diff --git a/hgext/formatsource.py b/hgext/formatsource.py
new file mode 100644
--- /dev/null
+++ b/hgext/formatsource.py
@@ -0,0 +1,331 @@
+# Copyright 2017 Octobus <contact@octobus.net>
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+"""help dealing with code source reformating
+
+The extension provides a way to run code-formatting tools in a way that avoids
+conflicts related to this formatting when merging/rebasing code across the
+reformatting.
+
+A new `format-source` command is provided, to apply code formatting tool on
+some specific files. This information is recorded into the repository and
+reused when merging. The client doing the merge needs the extension for this
+logic to kick in.
+
+Code formatting tools have to be registered in the configuration. The tool
+"name" will be used to identify a specific command accross all repositories.
+It is mapped to a command line that must output the formatted content on its
+standard output.
+
+For each tool a list of files affecting the result of the formatting can be
+configured with the "configpaths" suboption, which is read and registered at
+"hg format-source" time.  Any change in those files should trigger
+reformatting.
+
+Example::
+
+    [format-source]
+    json = python -m json.tool
+    clang = clang-format -style=Mozilla
+    clang:configpaths = .clang-format, .clang-format-ignore
+
+We do not support specifying the mapping of tool name to tool command in the
+repository itself for security reasons.
+
+The code formatting information is tracked in a .hg-format-source file at the
+root of the repository.
+
+Warning: There is no special logic handling renames so moving files to a
+directory not covered by the patterns used for the initial formatting will
+likely fail.
+"""
+
+from __future__ import absolute_import
+
+import json
+import tempfile
+
+from mercurial.i18n import _
+
+from mercurial import (
+    cmdutil,
+    commands,
+    encoding,
+    error,
+    extensions,
+    filemerge,
+    match,
+    merge,
+    registrar,
+    scmutil,
+    util,
+)
+
+cmdtable = {}
+command = registrar.command(cmdtable)
+
+configtable = {}
+configitem = registrar.configitem(configtable)
+configitem('format-source', '.*', default=None, generic=True)
+
+file_storage_path = '.hg-format-source'
+
+# Note for extension authors: ONLY specify testedwith = 'ships-with-hg-core' for
+# extensions which SHIP WITH MERCURIAL. Non-mainline extensions should
+# be specifying the version(s) of Mercurial they are tested with, or
+# leave the attribute unspecified.
+testedwith = 'ships-with-hg-core'
+
+@command('format-source',
+        [] + commands.walkopts + commands.commitopts + commands.commitopts2,
+        _('TOOL FILES+'))
+def cmd_format_source(ui, repo, tool, *pats, **opts):
+    """format source file using a registered tools
+
+    This command run TOOL on FILES and record this information in a commit to
+    help with future merge.
+
+    The actual command run for TOOL needs to be registered in the config. See
+    :hg:`help -e formatsource` for details.
+    """
+    if repo.getcwd():
+        msg = _("format-source must be run from repository root")
+        hint = _("cd %s") % repo.root
+        raise error.Abort(msg, hint=hint)
+
+    if not pats:
+        raise error.Abort(_('no files specified'))
+
+    # XXX We support glob pattern only for now, the recursive behavior of
+    # various others is a bit wonky.
+    for pattern in pats:
+        if not pattern.startswith('glob:'):
+            msg = _("format-source only supports explicit 'glob' patterns "
+                    "for now ('%s')")
+            msg %= pattern
+            hint = _('maybe try with "glob:%s"') % pattern
+            raise error.Abort(msg, hint=hint)
+
+    # lock the repo to make sure no content is changed
+    with repo.wlock():
+        # formating tool
+        if ' ' in tool:
+            raise error.Abort(_("tool name cannot contains space:"
+                                " '%s'") % tool)
+        shell_tool = repo.ui.config('format-source', tool)
+        tool_config_files = repo.ui.configlist('format-source',
+                                               '%s:configpaths' % tool)
+        if not shell_tool:
+            msg = _("unknow format tool: %s (no 'format-source.%s' config)")
+            raise error.Abort(msg % (tool, tool))
+        cmdutil.bailifchanged(repo)
+        cmdutil.checkunfinished(repo, commit=True)
+        wctx = repo[None]
+        # files to be formatted
+        matcher = scmutil.match(wctx, pats, opts)
+        # perform actual formatting
+        for filepath in wctx.matches(matcher):
+            flags = wctx.flags(filepath)
+            newcontent = run_tools(ui, repo.root, tool, shell_tool,
+                                   filepath, filepath)
+            # XXX we could do the whole commit in memory
+            with repo.wvfs(filepath, 'wb') as formatted_file:
+                formatted_file.write(newcontent)
+            wctx.filectx(filepath).setflags('l' in flags, 'x' in flags)
+
+        # update the storage to mark formated file as formatted
+        with repo.wvfs(file_storage_path, mode='ab') as storage:
+            for pattern in pats:
+                # XXX if pattern was relative, we need to reroot it from the
+                # repository root. For now we constrainted the command to run
+                # at the root of the repository.
+                data = {'tool': encoding.unifromlocal(tool),
+                        'pattern': encoding.unifromlocal(pattern)}
+                if tool_config_files:
+                    data['configpaths'] = [encoding.unifromlocal(path)
+                                           for path in tool_config_files]
+                entry = json.dumps(data, sort_keys=True)
+                assert '\n' not in entry
+                storage.write('%s\n' % entry)
+
+        if file_storage_path not in wctx:
+            storage_matcher = scmutil.match(wctx, ['path:' + file_storage_path])
+            cmdutil.add(ui, repo, storage_matcher, '', True)
+
+        # commit the whole
+        with repo.lock():
+            commit_patterns = ['path:' + file_storage_path]
+            commit_patterns.extend(pats)
+            return commands._docommit(ui, repo, *commit_patterns, **opts)
+
+def run_tools(ui, root, tool, cmd, filepath, filename):
+    """Run the a formatter tool on a specific file"""
+    env = encoding.environ.copy()
+    env['HG_FILENAME'] = filename
+    # XXX escape special character in filepath
+    format_cmd = "%s %s" % (cmd, filepath)
+    ui.debug('running %s\n' % format_cmd)
+    ui.pushbuffer(subproc=True)
+    try:
+        ui.system(format_cmd,
+                  environ=env,
+                  cwd=root,
+                  onerr=error.Abort,
+                  errprefix=tool)
+    finally:
+        newcontent = ui.popbuffer()
+    return newcontent
+
+def touched(repo, old_ctx, new_ctx, paths):
+    matcher = rootedmatch(repo, new_ctx, paths)
+    if any(path in new_ctx for path in paths):
+        status = old_ctx.status(other=new_ctx, match=matcher)
+        return bool(status.modified or status.added)
+    return False
+
+def formatted(repo, old_ctx, new_ctx):
+    """retrieve the list of formatted patterns between <old> and <new>
+
+    return a {'tool': [patterns]} mapping
+    """
+    new_formatting = {}
+    if touched(repo, old_ctx, new_ctx, [file_storage_path]):
+        # quick and dirty line diffing
+        # (the file is append only by contract)
+
+        new_lines = set(new_ctx[file_storage_path].data().splitlines())
+        old_lines = set()
+        if file_storage_path in old_ctx:
+            old_lines = set(old_ctx[file_storage_path].data().splitlines())
+        new_lines -= old_lines
+        for line in new_lines:
+            entry = json.loads(line)
+            def getkey(key):
+                return encoding.unitolocal(entry[key])
+            new_formatting.setdefault(getkey('tool'),
+                                      set()).add(getkey('pattern'))
+    if file_storage_path in old_ctx:
+        for line in old_ctx[file_storage_path].data().splitlines():
+            entry = json.loads(line)
+            if not entry.get('configpaths'):
+                continue
+            configpaths = [encoding.unitolocal(path) \
+                           for path in entry['configpaths']]
+            def getkey(key):
+                return encoding.unitolocal(entry[key])
+            if touched(repo, old_ctx, new_ctx, configpaths):
+                new_formatting.setdefault(getkey('tool'),
+                                          set()).add(getkey('pattern'))
+    return new_formatting
+
+def allformatted(repo, local, other, ancestor):
+    """return a mapping of formatting needed for all involved changeset
+    """
+
+    cachekey = (local.node, other.node(), ancestor.node())
+    cached = getattr(repo, '_formatting_cache', {}).get(cachekey)
+
+    if cached is not None:
+        return cached
+
+    local_formating = formatted(repo, ancestor, local)
+    other_formating = formatted(repo, ancestor, other)
+    full_formating = local_formating.copy()
+    for key, value in other_formating.iteritems():
+        if key in local_formating:
+            value = value | local_formating[key]
+        full_formating[key] = value
+
+    all = [
+        (local, local_formating),
+        (other, other_formating),
+        (ancestor, full_formating)
+    ]
+    for ctx, formatting in all:
+        for tool, patterns in formatting.iteritems():
+            formatting[tool] = rootedmatch(repo, ctx, patterns)
+
+    final = tuple(formatting for __, formatting in all)
+    getattr(repo, '_formatting_cache', {})[cachekey] = cached
+
+    return final
+
+def rootedmatch(repo, ctx, patterns):
+    """match patterns agains the root of a repository"""
+    # rework of basectx.match to ignore current working directory
+
+    # Only a case insensitive filesystem needs magic to translate user input
+    # to actual case in the filesystem.
+    icasefs = not util.fscasesensitive(repo.root)
+    if util.safehasattr(match, 'icasefsmatcher'): #< hg 4.3
+        if icasefs:
+            return match.icasefsmatcher(repo.root, repo.root, patterns,
+                                        default='glob', auditor=repo.auditor,
+                                        ctx=ctx)
+        else:
+            return match.match(repo.root, repo.root, patterns, default='glob',
+                               auditor=repo.auditor, ctx=ctx)
+    else:
+        return match.match(repo.root, repo.root, patterns, default='glob',
+                           auditor=repo.auditor, ctx=ctx, icasefs=icasefs)
+
+def apply_formating(repo, formatting, fctx):
+    """apply formatting to a file context (if applicable)"""
+    data = None
+    for tool, matcher in sorted(formatting.items()):
+        # matches?
+        if matcher(fctx.path()):
+            if data is None:
+                data = fctx.data()
+            shell_tool = repo.ui.config('format-source', tool)
+            if not shell_tool:
+                msg = _("format-source, no command defined for '%s',"
+                        " skipping formating: '%s'\n")
+                msg %= (tool, fctx.path())
+                repo.ui.warn(msg)
+                continue
+            with tempfile.NamedTemporaryFile(mode='wb') as f:
+                f.write(data)
+                f.flush()
+                data = run_tools(repo.ui, repo.root, tool, shell_tool,
+                                 f.name, fctx.path())
+    if data is not None:
+        fctx.data = lambda: data
+
+
+def wrap_filemerge(origfunc, premerge, repo, wctx, mynode, orig, fcd, fco,
+                     fca, *args, **kwargs):
+    """wrap the file merge logic to apply formatting on files that needs them"""
+    _update_filemerge_content(repo, fcd, fco, fca)
+    return origfunc(premerge, repo, wctx, mynode, orig, fcd, fco, fca,
+                    *args, **kwargs)
+
+def _update_filemerge_content(repo, fcd, fco, fca):
+    if fcd.isabsent() or fco.isabsent() or fca.isabsent():
+        return
+    local = fcd._changectx
+    other = fco._changectx
+    ances = fca._changectx
+    all = allformatted(repo, local, other, ances)
+    local_formating, other_formating, full_formating = all
+    apply_formating(repo, local_formating, fco)
+    apply_formating(repo, other_formating, fcd)
+    apply_formating(repo, full_formating, fca)
+
+    if 'data' in vars(fcd): # XXX hacky way to check if data overwritten
+        file_path = repo.wvfs.join(fcd.path())
+        with open(file_path, 'wb') as local_file:
+            local_file.write(fcd.data())
+
+def wrap_update(orig, repo, *args, **kwargs):
+    """install the formatting cache"""
+    repo._formatting_cache = {}
+    try:
+        return orig(repo, *args, **kwargs)
+    finally:
+        del repo._formatting_cache
+
+def uisetup(self):
+    extensions.wrapfunction(filemerge, '_filemerge', wrap_filemerge)
+    extensions.wrapfunction(merge, 'update', wrap_update)
diff --git a/tests/test-format-config.t b/tests/test-format-config.t
new file mode 100644
--- /dev/null
+++ b/tests/test-format-config.t
@@ -0,0 +1,288 @@
+
+Basic init
+
+  $ code_root=`dirname $TESTDIR`
+
+  $ cat << EOF >> $HGRCPATH
+  > [extensions]
+  > formatsource=
+  > rebase =
+  > strip =
+  > [format-source]
+  > json = $PYTHON $TESTDIR/testlib/json-pretty.py
+  > json:configpaths = .json-indent
+  > [default]
+  > format-source=--date '0 0'
+  > EOF
+  $ HGMERGE=:merge3
+
+  $ hg init test_repo
+  $ cd test_repo
+
+Commit various json file
+
+  $ mkdir dir-1
+  $ cat << EOF > dir-1/file-1.json
+  > {"key1": [42,53,78], "key2": [9,3,8,1], "key3": ["London", "Paris", "Tokyo"]}
+  > EOF
+  $ cat << EOF > dir-1/file-2.json
+  > {"key1": 1, "key2": [0, 1, 2, 3, 4, 5, 6, 7, 8, 9], "key3": [54]}
+  > EOF
+  $ hg add .
+  adding dir-1/file-1.json
+  adding dir-1/file-2.json
+  $ hg commit --message 'initial commit'
+
+format them (in multiple steps)
+
+  $ hg format-source --date '0 0' json glob:*/file-1.json -m 'format without config'
+  $ hg export
+  # HG changeset patch
+  # User test
+  # Date 0 0
+  #      Thu Jan 01 00:00:00 1970 +0000
+  # Node ID fb63bdd6edbf60d86f7aeaf0d806ecf6555c02eb
+  # Parent  103bbf4a41e9e9010c27ab49c158f99b176d4f3e
+  format without config
+  
+  diff -r 103bbf4a41e9 -r fb63bdd6edbf .hg-format-source
+  --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/.hg-format-source	Thu Jan 01 00:00:00 1970 +0000
+  @@ -0,0 +1,1 @@
+  +{"configpaths": [".json-indent"], "pattern": "glob:*/file-1.json", "tool": "json"}
+  diff -r 103bbf4a41e9 -r fb63bdd6edbf dir-1/file-1.json
+  --- a/dir-1/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/dir-1/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+  @@ -1,1 +1,18 @@
+  -{"key1": [42,53,78], "key2": [9,3,8,1], "key3": ["London", "Paris", "Tokyo"]}
+  +{
+  +    "key1": [
+  +        42,
+  +        53,
+  +        78
+  +    ],
+  +    "key2": [
+  +        9,
+  +        3,
+  +        8,
+  +        1
+  +    ],
+  +    "key3": [
+  +        "London",
+  +        "Paris",
+  +        "Tokyo"
+  +    ]
+  +}
+  $ echo 2 > .json-indent
+  $ hg add .json-indent
+  $ $PYTHON $TESTDIR/testlib/json-pretty.py < dir-1/file-1.json > tmp
+  $ mv tmp dir-1/file-1.json
+  $ hg diff
+  diff -r fb63bdd6edbf .json-indent
+  --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/.json-indent	Thu Jan 01 00:00:00 1970 +0000
+  @@ -0,0 +1,1 @@
+  +2
+  diff -r fb63bdd6edbf dir-1/file-1.json
+  --- a/dir-1/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/dir-1/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+  @@ -1,18 +1,18 @@
+   {
+  -    "key1": [
+  -        42,
+  -        53,
+  -        78
+  -    ],
+  -    "key2": [
+  -        9,
+  -        3,
+  -        8,
+  -        1
+  -    ],
+  -    "key3": [
+  -        "London",
+  -        "Paris",
+  -        "Tokyo"
+  -    ]
+  +  "key1": [
+  +    42,
+  +    53,
+  +    78
+  +  ],
+  +  "key2": [
+  +    9,
+  +    3,
+  +    8,
+  +    1
+  +  ],
+  +  "key3": [
+  +    "London",
+  +    "Paris",
+  +    "Tokyo"
+  +  ]
+   }
+  $ hg commit -m 'reformat with indent=2'
+  $ echo 1 > .json-indent
+  $ $PYTHON $TESTDIR/testlib/json-pretty.py < dir-1/file-1.json > tmp
+  $ mv tmp dir-1/file-1.json
+  $ hg diff
+  diff -r bacb7be97453 .json-indent
+  --- a/.json-indent	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/.json-indent	Thu Jan 01 00:00:00 1970 +0000
+  @@ -1,1 +1,1 @@
+  -2
+  +1
+  diff -r bacb7be97453 dir-1/file-1.json
+  --- a/dir-1/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/dir-1/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+  @@ -1,18 +1,18 @@
+   {
+  -  "key1": [
+  -    42,
+  -    53,
+  -    78
+  -  ],
+  -  "key2": [
+  -    9,
+  -    3,
+  -    8,
+  -    1
+  -  ],
+  -  "key3": [
+  -    "London",
+  -    "Paris",
+  -    "Tokyo"
+  -  ]
+  + "key1": [
+  +  42,
+  +  53,
+  +  78
+  + ],
+  + "key2": [
+  +  9,
+  +  3,
+  +  8,
+  +  1
+  + ],
+  + "key3": [
+  +  "London",
+  +  "Paris",
+  +  "Tokyo"
+  + ]
+   }
+  $ hg commit -m 'reformat with indent=1'
+
+Add changes on another branch
+
+  $ hg up 0
+  1 files updated, 0 files merged, 2 files removed, 0 files unresolved
+  $ cat << EOF > dir-1/file-1.json
+  > {"key1": [42,53,78,66], "key2": [9,3,8,1], "key3": ["London", "Paris", "Tokyo"]}
+  > EOF
+  $ cat << EOF > dir-1/file-2.json
+  > {"key1": 1, "key2": [0, 1, 2, 3, 4, 5, 6, 7, 8, 9], "key3": [54, 55]}
+  > EOF
+  $ hg commit -m 'some editions'
+  created new head
+
+Merge with "format without config"
+
+  $ hg log -G
+  @  changeset:   4:360af76de133
+  |  tag:         tip
+  |  parent:      0:103bbf4a41e9
+  |  user:        test
+  |  date:        Thu Jan 01 00:00:00 1970 +0000
+  |  summary:     some editions
+  |
+  | o  changeset:   3:fa670ec0f89c
+  | |  user:        test
+  | |  date:        Thu Jan 01 00:00:00 1970 +0000
+  | |  summary:     reformat with indent=1
+  | |
+  | o  changeset:   2:bacb7be97453
+  | |  user:        test
+  | |  date:        Thu Jan 01 00:00:00 1970 +0000
+  | |  summary:     reformat with indent=2
+  | |
+  | o  changeset:   1:fb63bdd6edbf
+  |/   user:        test
+  |    date:        Thu Jan 01 00:00:00 1970 +0000
+  |    summary:     format without config
+  |
+  o  changeset:   0:103bbf4a41e9
+     user:        test
+     date:        Thu Jan 01 00:00:00 1970 +0000
+     summary:     initial commit
+  
+  $ f --md5 dir-1/file-2.json
+  dir-1/file-2.json: md5=19be5044f37a12515b1bf92e2becf702
+  $ hg merge 1
+  merging dir-1/file-1.json
+  1 files updated, 1 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ f --md5 dir-1/file-2.json
+  dir-1/file-2.json: md5=19be5044f37a12515b1bf92e2becf702
+  $ hg diff -r 'p2()' dir-1/file-1.json
+  diff -r fb63bdd6edbf dir-1/file-1.json
+  --- a/dir-1/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/dir-1/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+  @@ -2,7 +2,8 @@
+       "key1": [
+           42,
+           53,
+  -        78
+  +        78,
+  +        66
+       ],
+       "key2": [
+           9,
+  $ hg commit -m 'merge #1'
+
+Merge with "format with indent=2"
+
+  $ hg merge 2
+  merging dir-1/file-1.json
+  1 files updated, 1 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ f --md5 dir-1/file-2.json
+  dir-1/file-2.json: md5=19be5044f37a12515b1bf92e2becf702
+  $ hg diff -r 'p2()' dir-1/file-1.json
+  diff -r bacb7be97453 dir-1/file-1.json
+  --- a/dir-1/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/dir-1/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+  @@ -2,7 +2,8 @@
+     "key1": [
+       42,
+       53,
+  -    78
+  +    78,
+  +    66
+     ],
+     "key2": [
+       9,
+  $ hg commit -m 'merge #2'
+
+Merge with indent=1
+
+  $ hg merge 3
+  merging dir-1/file-1.json
+  1 files updated, 1 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ f --md5 dir-1/file-2.json
+  dir-1/file-2.json: md5=19be5044f37a12515b1bf92e2becf702
+  $ hg diff -r 'p2()' dir-1/file-1.json
+  diff -r fa670ec0f89c dir-1/file-1.json
+  --- a/dir-1/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/dir-1/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+  @@ -2,7 +2,8 @@
+    "key1": [
+     42,
+     53,
+  -  78
+  +  78,
+  +  66
+    ],
+    "key2": [
+     9,
+  $ hg commit -m 'merge #3'
diff --git a/tests/test-format-source.t b/tests/test-format-source.t
new file mode 100644
--- /dev/null
+++ b/tests/test-format-source.t
@@ -0,0 +1,1376 @@
+
+Basic init
+
+  $ code_root=`dirname $TESTDIR`
+
+  $ cat << EOF >> $HGRCPATH
+  > [extensions]
+  > formatsource=
+  > rebase =
+  > strip =
+  > [format-source]
+  > json = $PYTHON -m json.tool
+  > [default]
+  > format-source=--date '0 0'
+  > [ui]
+  > merge = internal:merge3
+  > EOF
+
+  $ hg init test_repo
+  $ cd test_repo
+
+Commit various json file
+
+  $ cat << EOF > root-file.json
+  > {"key1": 1, "key2": [5,6,7,8], "key3": ["arthur", "babar", "celeste"]}
+  > EOF
+  $ mkdir dir-1
+  $ cat << EOF > dir-1/file-1.json
+  > {"key1": [42,53,78], "key2": [9,3,8,1], "key3": ["London", "Paris", "Tokyo"]}
+  > EOF
+  $ cat << EOF > dir-1/file-2.json
+  > {"key1": 1, "key2": [0, 1, 2, 3, 4, 5, 6, 7, 8, 9], "key3": [54]}
+  > EOF
+  $ mkdir dir-2
+  $ cat << EOF > dir-2/file-1.json
+  > {"key1": [44,50,82], "key2": [4, 8, -4, 5], "key3": ["Asia", "Europe"]}
+  > EOF
+  $ cat << EOF > dir-2/file-2.json
+  > {"key1": 6,
+  > "key2": [3, 5, -2, 3, 4, 11, 10, -4, 8, 9],
+  > "key3": [898, 32543, 2342]}
+  > EOF
+  $ cat << EOF > dir-2/file-3.json
+  > {"key1": "hello", "key2": [3, 1, 8, -1, 19, 2], "key3": "babar"}
+  > EOF
+  $ hg add .
+  adding dir-1/file-1.json
+  adding dir-1/file-2.json
+  adding dir-2/file-1.json
+  adding dir-2/file-2.json
+  adding dir-2/file-3.json
+  adding root-file.json
+  $ hg commit --message 'initial commit'
+
+format them (in multiple steps)
+
+  $ hg format-source --date '0 0' json glob:root-file.json -m 'format the root-file'
+  $ hg export
+  # HG changeset patch
+  # User test
+  # Date 0 0
+  #      Thu Jan 01 00:00:00 1970 +0000
+  # Node ID 4d5d2129291fb884920bc2dce4a5da973634a6e9
+  # Parent  f168d21ce7659a51518009b8aab78a222af2ddf8
+  format the root-file
+  
+  diff -r f168d21ce765 -r 4d5d2129291f .hg-format-source
+  --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/.hg-format-source	Thu Jan 01 00:00:00 1970 +0000
+  @@ -0,0 +1,1 @@
+  +{"pattern": "glob:root-file.json", "tool": "json"}
+  diff -r f168d21ce765 -r 4d5d2129291f root-file.json
+  --- a/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  @@ -1,1 +1,14 @@
+  -{"key1": 1, "key2": [5,6,7,8], "key3": ["arthur", "babar", "celeste"]}
+  +{
+  +    "key1": 1,
+  +    "key2": [
+  +        5,
+  +        6,
+  +        7,
+  +        8
+  +    ],
+  +    "key3": [
+  +        "arthur",
+  +        "babar",
+  +        "celeste"
+  +    ]
+  +}
+  $ hg format-source --date '0 0' json 'glob:dir-1/**' -m 'format dir1 as a whole'
+  $ hg export
+  # HG changeset patch
+  # User test
+  # Date 0 0
+  #      Thu Jan 01 00:00:00 1970 +0000
+  # Node ID 279459a606de25c75ca2e2984a607ddf4a88ac29
+  # Parent  4d5d2129291fb884920bc2dce4a5da973634a6e9
+  format dir1 as a whole
+  
+  diff -r 4d5d2129291f -r 279459a606de .hg-format-source
+  --- a/.hg-format-source	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/.hg-format-source	Thu Jan 01 00:00:00 1970 +0000
+  @@ -1,1 +1,2 @@
+   {"pattern": "glob:root-file.json", "tool": "json"}
+  +{"pattern": "glob:dir-1/**", "tool": "json"}
+  diff -r 4d5d2129291f -r 279459a606de dir-1/file-1.json
+  --- a/dir-1/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/dir-1/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+  @@ -1,1 +1,18 @@
+  -{"key1": [42,53,78], "key2": [9,3,8,1], "key3": ["London", "Paris", "Tokyo"]}
+  +{
+  +    "key1": [
+  +        42,
+  +        53,
+  +        78
+  +    ],
+  +    "key2": [
+  +        9,
+  +        3,
+  +        8,
+  +        1
+  +    ],
+  +    "key3": [
+  +        "London",
+  +        "Paris",
+  +        "Tokyo"
+  +    ]
+  +}
+  diff -r 4d5d2129291f -r 279459a606de dir-1/file-2.json
+  --- a/dir-1/file-2.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/dir-1/file-2.json	Thu Jan 01 00:00:00 1970 +0000
+  @@ -1,1 +1,18 @@
+  -{"key1": 1, "key2": [0, 1, 2, 3, 4, 5, 6, 7, 8, 9], "key3": [54]}
+  +{
+  +    "key1": 1,
+  +    "key2": [
+  +        0,
+  +        1,
+  +        2,
+  +        3,
+  +        4,
+  +        5,
+  +        6,
+  +        7,
+  +        8,
+  +        9
+  +    ],
+  +    "key3": [
+  +        54
+  +    ]
+  +}
+  $ hg format-source --date '0 0' json glob:dir-2/file-1.json glob:dir-2/file-3.json -m 'format some dir2 with explicite pattern'
+  $ hg export
+  # HG changeset patch
+  # User test
+  # Date 0 0
+  #      Thu Jan 01 00:00:00 1970 +0000
+  # Node ID d29e366c2a4b03cc2c09c0581fed00147b5bbebc
+  # Parent  279459a606de25c75ca2e2984a607ddf4a88ac29
+  format some dir2 with explicite pattern
+  
+  diff -r 279459a606de -r d29e366c2a4b .hg-format-source
+  --- a/.hg-format-source	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/.hg-format-source	Thu Jan 01 00:00:00 1970 +0000
+  @@ -1,2 +1,4 @@
+   {"pattern": "glob:root-file.json", "tool": "json"}
+   {"pattern": "glob:dir-1/**", "tool": "json"}
+  +{"pattern": "glob:dir-2/file-1.json", "tool": "json"}
+  +{"pattern": "glob:dir-2/file-3.json", "tool": "json"}
+  diff -r 279459a606de -r d29e366c2a4b dir-2/file-1.json
+  --- a/dir-2/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/dir-2/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+  @@ -1,1 +1,17 @@
+  -{"key1": [44,50,82], "key2": [4, 8, -4, 5], "key3": ["Asia", "Europe"]}
+  +{
+  +    "key1": [
+  +        44,
+  +        50,
+  +        82
+  +    ],
+  +    "key2": [
+  +        4,
+  +        8,
+  +        -4,
+  +        5
+  +    ],
+  +    "key3": [
+  +        "Asia",
+  +        "Europe"
+  +    ]
+  +}
+  diff -r 279459a606de -r d29e366c2a4b dir-2/file-3.json
+  --- a/dir-2/file-3.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/dir-2/file-3.json	Thu Jan 01 00:00:00 1970 +0000
+  @@ -1,1 +1,12 @@
+  -{"key1": "hello", "key2": [3, 1, 8, -1, 19, 2], "key3": "babar"}
+  +{
+  +    "key1": "hello",
+  +    "key2": [
+  +        3,
+  +        1,
+  +        8,
+  +        -1,
+  +        19,
+  +        2
+  +    ],
+  +    "key3": "babar"
+  +}
+  $ hg log -G
+  @  changeset:   3:d29e366c2a4b
+  |  tag:         tip
+  |  user:        test
+  |  date:        Thu Jan 01 00:00:00 1970 +0000
+  |  summary:     format some dir2 with explicite pattern
+  |
+  o  changeset:   2:279459a606de
+  |  user:        test
+  |  date:        Thu Jan 01 00:00:00 1970 +0000
+  |  summary:     format dir1 as a whole
+  |
+  o  changeset:   1:4d5d2129291f
+  |  user:        test
+  |  date:        Thu Jan 01 00:00:00 1970 +0000
+  |  summary:     format the root-file
+  |
+  o  changeset:   0:f168d21ce765
+     user:        test
+     date:        Thu Jan 01 00:00:00 1970 +0000
+     summary:     initial commit
+  
+
+Test merging
+============
+
+Simple case, the root file
+
+  $ hg up 0
+  5 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ cat << EOF > root-file.json
+  > {"key1": 1, "key2": [5,6,7,8], "key3": ["arthur", "babar", "celeste", "pomme"]}
+  > EOF
+  $ hg branch B_A
+  marked working directory as branch B_A
+  (branches are permanent and global, did you want a bookmark?)
+  $ hg ci -m 'update root-file'
+  $ hg merge 1
+  merging root-file.json
+  1 files updated, 1 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg diff -r 'p1()'
+  diff -r 86b2ca2ae552 .hg-format-source
+  --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/.hg-format-source	* (glob)
+  @@ -0,0 +1,1 @@
+  +{"pattern": "glob:root-file.json", "tool": "json"}
+  diff -r 86b2ca2ae552 root-file.json
+  --- a/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/root-file.json	* (glob)
+  @@ -1,1 +1,15 @@
+  -{"key1": 1, "key2": [5,6,7,8], "key3": ["arthur", "babar", "celeste", "pomme"]}
+  +{
+  +    "key1": 1,
+  +    "key2": [
+  +        5,
+  +        6,
+  +        7,
+  +        8
+  +    ],
+  +    "key3": [
+  +        "arthur",
+  +        "babar",
+  +        "celeste",
+  +        "pomme"
+  +    ]
+  +}
+  $ hg diff -r 'p2()'
+  diff -r 4d5d2129291f root-file.json
+  --- a/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/root-file.json	* (glob)
+  @@ -9,6 +9,7 @@
+       "key3": [
+           "arthur",
+           "babar",
+  -        "celeste"
+  +        "celeste",
+  +        "pomme"
+       ]
+   }
+
+Same from the other direction
+
+  $ hg up -C 1
+  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg merge B_A
+  merging root-file.json
+  0 files updated, 1 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg diff -r 'p1()'
+  diff -r 4d5d2129291f root-file.json
+  --- a/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/root-file.json	* (glob)
+  @@ -9,6 +9,7 @@
+       "key3": [
+           "arthur",
+           "babar",
+  -        "celeste"
+  +        "celeste",
+  +        "pomme"
+       ]
+   }
+  $ hg diff -r 'p2()'
+  diff -r 86b2ca2ae552 .hg-format-source
+  --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/.hg-format-source	* (glob)
+  @@ -0,0 +1,1 @@
+  +{"pattern": "glob:root-file.json", "tool": "json"}
+  diff -r 86b2ca2ae552 root-file.json
+  --- a/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/root-file.json	* (glob)
+  @@ -1,1 +1,15 @@
+  -{"key1": 1, "key2": [5,6,7,8], "key3": ["arthur", "babar", "celeste", "pomme"]}
+  +{
+  +    "key1": 1,
+  +    "key2": [
+  +        5,
+  +        6,
+  +        7,
+  +        8
+  +    ],
+  +    "key3": [
+  +        "arthur",
+  +        "babar",
+  +        "celeste",
+  +        "pomme"
+  +    ]
+  +}
+
+Merging change on both side:
+
+  $ hg up -C 1
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg branch B_B
+  marked working directory as branch B_B
+
+  $ echo '{"key1": 4, "key2": [5,6,7,8], "key3": ["arthur", "babar", "celeste"]}' | $PYTHON -m json.tool > root-file.json
+  $ hg ci -m 'update key 1'
+  $ hg merge B_A
+  merging root-file.json
+  0 files updated, 1 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg diff -r 'p1()'
+  diff -r 217d6c23789b root-file.json
+  --- a/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/root-file.json	* (glob)
+  @@ -9,6 +9,7 @@
+       "key3": [
+           "arthur",
+           "babar",
+  -        "celeste"
+  +        "celeste",
+  +        "pomme"
+       ]
+   }
+  $ hg diff -r 'p2()'
+  diff -r 86b2ca2ae552 .hg-format-source
+  --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/.hg-format-source	* (glob)
+  @@ -0,0 +1,1 @@
+  +{"pattern": "glob:root-file.json", "tool": "json"}
+  diff -r 86b2ca2ae552 root-file.json
+  --- a/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/root-file.json	* (glob)
+  @@ -1,1 +1,15 @@
+  -{"key1": 1, "key2": [5,6,7,8], "key3": ["arthur", "babar", "celeste", "pomme"]}
+  +{
+  +    "key1": 4,
+  +    "key2": [
+  +        5,
+  +        6,
+  +        7,
+  +        8
+  +    ],
+  +    "key3": [
+  +        "arthur",
+  +        "babar",
+  +        "celeste",
+  +        "pomme"
+  +    ]
+  +}
+
+Merging with conflict
+
+  $ hg up -C B_B
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+
+  $ echo '{"key1": 4, "key2": [5,6,7,8], "key3": ["arthur", "babar", "celeste", "Flore"]}' | $PYTHON -m json.tool > root-file.json
+  $ hg ci -m 'conflicting update'
+  $ hg merge B_A
+  merging root-file.json
+  warning: conflicts while merging root-file.json! (edit, then use 'hg resolve --mark')
+  0 files updated, 0 files merged, 0 files removed, 1 files unresolved
+  use 'hg resolve' to retry unresolved file merges or 'hg merge --abort' to abandon
+  [1]
+  $ hg resolve -l
+  U root-file.json
+  $ hg diff -r 'p1()'
+  diff -r 654f4c32c118 root-file.json
+  --- a/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/root-file.json	* (glob)
+  @@ -9,7 +9,14 @@
+       "key3": [
+           "arthur",
+           "babar",
+  +<<<<<<< working copy: 654f4c32c118 B_B - test: conflicting update
+           "celeste",
+           "Flore"
+  +||||||| base
+  +        "celeste"
+  +=======
+  +        "celeste",
+  +        "pomme"
+  +>>>>>>> merge rev:    86b2ca2ae552 B_A - test: update root-file
+       ]
+   }
+  $ hg diff -r 'p2()'
+  diff -r 86b2ca2ae552 .hg-format-source
+  --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/.hg-format-source	* (glob)
+  @@ -0,0 +1,1 @@
+  +{"pattern": "glob:root-file.json", "tool": "json"}
+  diff -r 86b2ca2ae552 root-file.json
+  --- a/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/root-file.json	* (glob)
+  @@ -1,1 +1,22 @@
+  -{"key1": 1, "key2": [5,6,7,8], "key3": ["arthur", "babar", "celeste", "pomme"]}
+  +{
+  +    "key1": 4,
+  +    "key2": [
+  +        5,
+  +        6,
+  +        7,
+  +        8
+  +    ],
+  +    "key3": [
+  +        "arthur",
+  +        "babar",
+  +<<<<<<< working copy: 654f4c32c118 B_B - test: conflicting update
+  +        "celeste",
+  +        "Flore"
+  +||||||| base
+  +        "celeste"
+  +=======
+  +        "celeste",
+  +        "pomme"
+  +>>>>>>> merge rev:    86b2ca2ae552 B_A - test: update root-file
+  +    ]
+  +}
+
+Test merge with no reformating needed
+-------------------------------------
+
+  $ hg up -C B_B
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg merge default
+  5 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg diff -r 'p2()'
+  diff -r d29e366c2a4b root-file.json
+  --- a/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/root-file.json	* (glob)
+  @@ -1,5 +1,5 @@
+   {
+  -    "key1": 1,
+  +    "key1": 4,
+       "key2": [
+           5,
+           6,
+  @@ -9,6 +9,7 @@
+       "key3": [
+           "arthur",
+           "babar",
+  -        "celeste"
+  +        "celeste",
+  +        "Flore"
+       ]
+   }
+
+Test rebase
+-----------
+
+  $ hg up -C B_A
+  5 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ hg rebase -d default
+  rebasing 4:86b2ca2ae552 "update root-file"
+  merging root-file.json
+  saved backup bundle to $TESTTMP/test_repo/.hg/strip-backup/86b2ca2ae552-2a92ee50-*.hg (glob)
+  $ hg export
+  # HG changeset patch
+  # User test
+  # Date 0 0
+  #      Thu Jan 01 00:00:00 1970 +0000
+  # Node ID 180a5b3a41142f9e2f40e773ed299bc996442266
+  # Parent  d29e366c2a4b03cc2c09c0581fed00147b5bbebc
+  update root-file
+  
+  diff -r d29e366c2a4b -r 180a5b3a4114 root-file.json
+  --- a/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  @@ -9,6 +9,7 @@
+       "key3": [
+           "arthur",
+           "babar",
+  -        "celeste"
+  +        "celeste",
+  +        "pomme"
+       ]
+   }
+  $ hg log -G
+  @  changeset:   6:180a5b3a4114
+  |  tag:         tip
+  |  parent:      3:d29e366c2a4b
+  |  user:        test
+  |  date:        Thu Jan 01 00:00:00 1970 +0000
+  |  summary:     update root-file
+  |
+  | o  changeset:   5:654f4c32c118
+  | |  branch:      B_B
+  | |  user:        test
+  | |  date:        Thu Jan 01 00:00:00 1970 +0000
+  | |  summary:     conflicting update
+  | |
+  | o  changeset:   4:217d6c23789b
+  | |  branch:      B_B
+  | |  parent:      1:4d5d2129291f
+  | |  user:        test
+  | |  date:        Thu Jan 01 00:00:00 1970 +0000
+  | |  summary:     update key 1
+  | |
+  o |  changeset:   3:d29e366c2a4b
+  | |  user:        test
+  | |  date:        Thu Jan 01 00:00:00 1970 +0000
+  | |  summary:     format some dir2 with explicite pattern
+  | |
+  o |  changeset:   2:279459a606de
+  |/   user:        test
+  |    date:        Thu Jan 01 00:00:00 1970 +0000
+  |    summary:     format dir1 as a whole
+  |
+  o  changeset:   1:4d5d2129291f
+  |  user:        test
+  |  date:        Thu Jan 01 00:00:00 1970 +0000
+  |  summary:     format the root-file
+  |
+  o  changeset:   0:f168d21ce765
+     user:        test
+     date:        Thu Jan 01 00:00:00 1970 +0000
+     summary:     initial commit
+  
+
+Test update
+-----------
+
+  $ hg up -C 0
+  5 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ cat << EOF > root-file.json
+  > {"key1": 1, "key2": [4,6,7,8], "key3": ["arthur", "babar", "celeste"]}
+  > EOF
+  $ hg diff
+  diff -r f168d21ce765 root-file.json
+  --- a/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/root-file.json	* (glob)
+  @@ -1,1 +1,1 @@
+  -{"key1": 1, "key2": [5,6,7,8], "key3": ["arthur", "babar", "celeste"]}
+  +{"key1": 1, "key2": [4,6,7,8], "key3": ["arthur", "babar", "celeste"]}
+  $ hg up --merge
+  merging root-file.json
+  5 files updated, 1 files merged, 0 files removed, 0 files unresolved
+  $ hg diff
+  diff -r 180a5b3a4114 root-file.json
+  --- a/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/root-file.json	* (glob)
+  @@ -1,7 +1,7 @@
+   {
+       "key1": 1,
+       "key2": [
+  -        5,
+  +        4,
+           6,
+           7,
+           8
+
+
+Test multiple change to multiple file accross multiple revisions
+----------------------------------------------------------------
+
+modify one side
+
+  $ hg up -C default
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ echo '{"key1": [42,53,76], "key2": [9,3,8,1], "key3": ["London", "Paris", "Tokyo"]}' | $PYTHON -m json.tool > dir-1/file-1.json
+  $ hg diff
+  diff -r 180a5b3a4114 dir-1/file-1.json
+  --- a/dir-1/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/dir-1/file-1.json	* (glob)
+  @@ -2,7 +2,7 @@
+       "key1": [
+           42,
+           53,
+  -        78
+  +        76
+       ],
+       "key2": [
+           9,
+  $ hg ci -m 'update d1f1'
+  $ echo '{"key1": 4, "key2": [0, 1, 2, 3, 4, 5, 6, 7, 8, 9], "key3": [54]}' | $PYTHON -m json.tool > dir-1/file-2.json
+  $ echo '{"key1": [44,50,86], "key2": [4, 8, -4, 5], "key3": ["Asia", "Europe", "Oceania"]}' | $PYTHON -m json.tool > dir-2/file-1.json
+  $ hg diff
+  diff -r 0daf45f1c8ad dir-1/file-2.json
+  --- a/dir-1/file-2.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/dir-1/file-2.json	* (glob)
+  @@ -1,5 +1,5 @@
+   {
+  -    "key1": 1,
+  +    "key1": 4,
+       "key2": [
+           0,
+           1,
+  diff -r 0daf45f1c8ad dir-2/file-1.json
+  --- a/dir-2/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/dir-2/file-1.json	* (glob)
+  @@ -2,7 +2,7 @@
+       "key1": [
+           44,
+           50,
+  -        82
+  +        86
+       ],
+       "key2": [
+           4,
+  @@ -12,6 +12,7 @@
+       ],
+       "key3": [
+           "Asia",
+  -        "Europe"
+  +        "Europe",
+  +        "Oceania"
+       ]
+   }
+  $ hg ci -m 'update d1f2 and d2f1'
+
+modify the other side
+
+  $ hg up -C B_B
+  6 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ echo '{"key1": 1337, "key2": [5,6,7,8], "key3": ["arthur", "babar", "celeste"]}' | $PYTHON -m json.tool > root-file.json
+  $ echo '{"key1": 4, "key2": [0, 1, 3, 3, 7, 5, 9, 7, 8, 9], "key3": [54]}' > dir-1/file-2.json
+  $ hg diff
+  diff -r 654f4c32c118 dir-1/file-2.json
+  --- a/dir-1/file-2.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/dir-1/file-2.json	* (glob)
+  @@ -1,1 +1,1 @@
+  -{"key1": 1, "key2": [0, 1, 2, 3, 4, 5, 6, 7, 8, 9], "key3": [54]}
+  +{"key1": 4, "key2": [0, 1, 3, 3, 7, 5, 9, 7, 8, 9], "key3": [54]}
+  diff -r 654f4c32c118 root-file.json
+  --- a/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/root-file.json	* (glob)
+  @@ -1,5 +1,5 @@
+   {
+  -    "key1": 4,
+  +    "key1": 1337,
+       "key2": [
+           5,
+           6,
+  @@ -9,7 +9,6 @@
+       "key3": [
+           "arthur",
+           "babar",
+  -        "celeste",
+  -        "Flore"
+  +        "celeste"
+       ]
+   }
+  $ hg ci -m 'update root-file and d1f2'
+  $ cat << EOF > dir-2/file-2.json
+  > {"key1": 6,
+  > "key2": [3, 5, -2, 3, 4, 11, 10, -4, 8, 9],
+  > "key3": [898, 32543, 2336]}
+  > EOF
+  $ echo '{"key1": "hello", "key2": [6, 1, 8, -1, 19, 2], "key3": "Babar"}' > dir-2/file-3.json
+  $ hg diff
+  diff -r b3816cb249a4 dir-2/file-2.json
+  --- a/dir-2/file-2.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/dir-2/file-2.json	* (glob)
+  @@ -1,3 +1,3 @@
+   {"key1": 6,
+   "key2": [3, 5, -2, 3, 4, 11, 10, -4, 8, 9],
+  -"key3": [898, 32543, 2342]}
+  +"key3": [898, 32543, 2336]}
+  diff -r b3816cb249a4 dir-2/file-3.json
+  --- a/dir-2/file-3.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/dir-2/file-3.json	* (glob)
+  @@ -1,1 +1,1 @@
+  -{"key1": "hello", "key2": [3, 1, 8, -1, 19, 2], "key3": "babar"}
+  +{"key1": "hello", "key2": [6, 1, 8, -1, 19, 2], "key3": "Babar"}
+  $ hg ci -m 'update d2f2 and d2f3'
+
+Check output before leaping
+
+  $ hg log -Gp -r 'sort(all(), "topo")'
+  @  changeset:   10:db645e78c8a3
+  |  branch:      B_B
+  |  tag:         tip
+  |  user:        test
+  |  date:        Thu Jan 01 00:00:00 1970 +0000
+  |  summary:     update d2f2 and d2f3
+  |
+  |  diff -r b3816cb249a4 -r db645e78c8a3 dir-2/file-2.json
+  |  --- a/dir-2/file-2.json	Thu Jan 01 00:00:00 1970 +0000
+  |  +++ b/dir-2/file-2.json	Thu Jan 01 00:00:00 1970 +0000
+  |  @@ -1,3 +1,3 @@
+  |   {"key1": 6,
+  |   "key2": [3, 5, -2, 3, 4, 11, 10, -4, 8, 9],
+  |  -"key3": [898, 32543, 2342]}
+  |  +"key3": [898, 32543, 2336]}
+  |  diff -r b3816cb249a4 -r db645e78c8a3 dir-2/file-3.json
+  |  --- a/dir-2/file-3.json	Thu Jan 01 00:00:00 1970 +0000
+  |  +++ b/dir-2/file-3.json	Thu Jan 01 00:00:00 1970 +0000
+  |  @@ -1,1 +1,1 @@
+  |  -{"key1": "hello", "key2": [3, 1, 8, -1, 19, 2], "key3": "babar"}
+  |  +{"key1": "hello", "key2": [6, 1, 8, -1, 19, 2], "key3": "Babar"}
+  |
+  o  changeset:   9:b3816cb249a4
+  |  branch:      B_B
+  |  parent:      5:654f4c32c118
+  |  user:        test
+  |  date:        Thu Jan 01 00:00:00 1970 +0000
+  |  summary:     update root-file and d1f2
+  |
+  |  diff -r 654f4c32c118 -r b3816cb249a4 dir-1/file-2.json
+  |  --- a/dir-1/file-2.json	Thu Jan 01 00:00:00 1970 +0000
+  |  +++ b/dir-1/file-2.json	Thu Jan 01 00:00:00 1970 +0000
+  |  @@ -1,1 +1,1 @@
+  |  -{"key1": 1, "key2": [0, 1, 2, 3, 4, 5, 6, 7, 8, 9], "key3": [54]}
+  |  +{"key1": 4, "key2": [0, 1, 3, 3, 7, 5, 9, 7, 8, 9], "key3": [54]}
+  |  diff -r 654f4c32c118 -r b3816cb249a4 root-file.json
+  |  --- a/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  |  +++ b/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  |  @@ -1,5 +1,5 @@
+  |   {
+  |  -    "key1": 4,
+  |  +    "key1": 1337,
+  |       "key2": [
+  |           5,
+  |           6,
+  |  @@ -9,7 +9,6 @@
+  |       "key3": [
+  |           "arthur",
+  |           "babar",
+  |  -        "celeste",
+  |  -        "Flore"
+  |  +        "celeste"
+  |       ]
+  |   }
+  |
+  o  changeset:   5:654f4c32c118
+  |  branch:      B_B
+  |  user:        test
+  |  date:        Thu Jan 01 00:00:00 1970 +0000
+  |  summary:     conflicting update
+  |
+  |  diff -r 217d6c23789b -r 654f4c32c118 root-file.json
+  |  --- a/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  |  +++ b/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  |  @@ -9,6 +9,7 @@
+  |       "key3": [
+  |           "arthur",
+  |           "babar",
+  |  -        "celeste"
+  |  +        "celeste",
+  |  +        "Flore"
+  |       ]
+  |   }
+  |
+  o  changeset:   4:217d6c23789b
+  |  branch:      B_B
+  |  parent:      1:4d5d2129291f
+  |  user:        test
+  |  date:        Thu Jan 01 00:00:00 1970 +0000
+  |  summary:     update key 1
+  |
+  |  diff -r 4d5d2129291f -r 217d6c23789b root-file.json
+  |  --- a/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  |  +++ b/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  |  @@ -1,5 +1,5 @@
+  |   {
+  |  -    "key1": 1,
+  |  +    "key1": 4,
+  |       "key2": [
+  |           5,
+  |           6,
+  |
+  | o  changeset:   8:73175e0546e6
+  | |  user:        test
+  | |  date:        Thu Jan 01 00:00:00 1970 +0000
+  | |  summary:     update d1f2 and d2f1
+  | |
+  | |  diff -r 0daf45f1c8ad -r 73175e0546e6 dir-1/file-2.json
+  | |  --- a/dir-1/file-2.json	Thu Jan 01 00:00:00 1970 +0000
+  | |  +++ b/dir-1/file-2.json	Thu Jan 01 00:00:00 1970 +0000
+  | |  @@ -1,5 +1,5 @@
+  | |   {
+  | |  -    "key1": 1,
+  | |  +    "key1": 4,
+  | |       "key2": [
+  | |           0,
+  | |           1,
+  | |  diff -r 0daf45f1c8ad -r 73175e0546e6 dir-2/file-1.json
+  | |  --- a/dir-2/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+  | |  +++ b/dir-2/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+  | |  @@ -2,7 +2,7 @@
+  | |       "key1": [
+  | |           44,
+  | |           50,
+  | |  -        82
+  | |  +        86
+  | |       ],
+  | |       "key2": [
+  | |           4,
+  | |  @@ -12,6 +12,7 @@
+  | |       ],
+  | |       "key3": [
+  | |           "Asia",
+  | |  -        "Europe"
+  | |  +        "Europe",
+  | |  +        "Oceania"
+  | |       ]
+  | |   }
+  | |
+  | o  changeset:   7:0daf45f1c8ad
+  | |  user:        test
+  | |  date:        Thu Jan 01 00:00:00 1970 +0000
+  | |  summary:     update d1f1
+  | |
+  | |  diff -r 180a5b3a4114 -r 0daf45f1c8ad dir-1/file-1.json
+  | |  --- a/dir-1/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+  | |  +++ b/dir-1/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+  | |  @@ -2,7 +2,7 @@
+  | |       "key1": [
+  | |           42,
+  | |           53,
+  | |  -        78
+  | |  +        76
+  | |       ],
+  | |       "key2": [
+  | |           9,
+  | |
+  | o  changeset:   6:180a5b3a4114
+  | |  parent:      3:d29e366c2a4b
+  | |  user:        test
+  | |  date:        Thu Jan 01 00:00:00 1970 +0000
+  | |  summary:     update root-file
+  | |
+  | |  diff -r d29e366c2a4b -r 180a5b3a4114 root-file.json
+  | |  --- a/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  | |  +++ b/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  | |  @@ -9,6 +9,7 @@
+  | |       "key3": [
+  | |           "arthur",
+  | |           "babar",
+  | |  -        "celeste"
+  | |  +        "celeste",
+  | |  +        "pomme"
+  | |       ]
+  | |   }
+  | |
+  | o  changeset:   3:d29e366c2a4b
+  | |  user:        test
+  | |  date:        Thu Jan 01 00:00:00 1970 +0000
+  | |  summary:     format some dir2 with explicite pattern
+  | |
+  | |  diff -r 279459a606de -r d29e366c2a4b .hg-format-source
+  | |  --- a/.hg-format-source	Thu Jan 01 00:00:00 1970 +0000
+  | |  +++ b/.hg-format-source	Thu Jan 01 00:00:00 1970 +0000
+  | |  @@ -1,2 +1,4 @@
+  | |   {"pattern": "glob:root-file.json", "tool": "json"}
+  | |   {"pattern": "glob:dir-1/**", "tool": "json"}
+  | |  +{"pattern": "glob:dir-2/file-1.json", "tool": "json"}
+  | |  +{"pattern": "glob:dir-2/file-3.json", "tool": "json"}
+  | |  diff -r 279459a606de -r d29e366c2a4b dir-2/file-1.json
+  | |  --- a/dir-2/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+  | |  +++ b/dir-2/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+  | |  @@ -1,1 +1,17 @@
+  | |  -{"key1": [44,50,82], "key2": [4, 8, -4, 5], "key3": ["Asia", "Europe"]}
+  | |  +{
+  | |  +    "key1": [
+  | |  +        44,
+  | |  +        50,
+  | |  +        82
+  | |  +    ],
+  | |  +    "key2": [
+  | |  +        4,
+  | |  +        8,
+  | |  +        -4,
+  | |  +        5
+  | |  +    ],
+  | |  +    "key3": [
+  | |  +        "Asia",
+  | |  +        "Europe"
+  | |  +    ]
+  | |  +}
+  | |  diff -r 279459a606de -r d29e366c2a4b dir-2/file-3.json
+  | |  --- a/dir-2/file-3.json	Thu Jan 01 00:00:00 1970 +0000
+  | |  +++ b/dir-2/file-3.json	Thu Jan 01 00:00:00 1970 +0000
+  | |  @@ -1,1 +1,12 @@
+  | |  -{"key1": "hello", "key2": [3, 1, 8, -1, 19, 2], "key3": "babar"}
+  | |  +{
+  | |  +    "key1": "hello",
+  | |  +    "key2": [
+  | |  +        3,
+  | |  +        1,
+  | |  +        8,
+  | |  +        -1,
+  | |  +        19,
+  | |  +        2
+  | |  +    ],
+  | |  +    "key3": "babar"
+  | |  +}
+  | |
+  | o  changeset:   2:279459a606de
+  |/   user:        test
+  |    date:        Thu Jan 01 00:00:00 1970 +0000
+  |    summary:     format dir1 as a whole
+  |
+  |    diff -r 4d5d2129291f -r 279459a606de .hg-format-source
+  |    --- a/.hg-format-source	Thu Jan 01 00:00:00 1970 +0000
+  |    +++ b/.hg-format-source	Thu Jan 01 00:00:00 1970 +0000
+  |    @@ -1,1 +1,2 @@
+  |     {"pattern": "glob:root-file.json", "tool": "json"}
+  |    +{"pattern": "glob:dir-1/**", "tool": "json"}
+  |    diff -r 4d5d2129291f -r 279459a606de dir-1/file-1.json
+  |    --- a/dir-1/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+  |    +++ b/dir-1/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+  |    @@ -1,1 +1,18 @@
+  |    -{"key1": [42,53,78], "key2": [9,3,8,1], "key3": ["London", "Paris", "Tokyo"]}
+  |    +{
+  |    +    "key1": [
+  |    +        42,
+  |    +        53,
+  |    +        78
+  |    +    ],
+  |    +    "key2": [
+  |    +        9,
+  |    +        3,
+  |    +        8,
+  |    +        1
+  |    +    ],
+  |    +    "key3": [
+  |    +        "London",
+  |    +        "Paris",
+  |    +        "Tokyo"
+  |    +    ]
+  |    +}
+  |    diff -r 4d5d2129291f -r 279459a606de dir-1/file-2.json
+  |    --- a/dir-1/file-2.json	Thu Jan 01 00:00:00 1970 +0000
+  |    +++ b/dir-1/file-2.json	Thu Jan 01 00:00:00 1970 +0000
+  |    @@ -1,1 +1,18 @@
+  |    -{"key1": 1, "key2": [0, 1, 2, 3, 4, 5, 6, 7, 8, 9], "key3": [54]}
+  |    +{
+  |    +    "key1": 1,
+  |    +    "key2": [
+  |    +        0,
+  |    +        1,
+  |    +        2,
+  |    +        3,
+  |    +        4,
+  |    +        5,
+  |    +        6,
+  |    +        7,
+  |    +        8,
+  |    +        9
+  |    +    ],
+  |    +    "key3": [
+  |    +        54
+  |    +    ]
+  |    +}
+  |
+  o  changeset:   1:4d5d2129291f
+  |  user:        test
+  |  date:        Thu Jan 01 00:00:00 1970 +0000
+  |  summary:     format the root-file
+  |
+  |  diff -r f168d21ce765 -r 4d5d2129291f .hg-format-source
+  |  --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  |  +++ b/.hg-format-source	Thu Jan 01 00:00:00 1970 +0000
+  |  @@ -0,0 +1,1 @@
+  |  +{"pattern": "glob:root-file.json", "tool": "json"}
+  |  diff -r f168d21ce765 -r 4d5d2129291f root-file.json
+  |  --- a/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  |  +++ b/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  |  @@ -1,1 +1,14 @@
+  |  -{"key1": 1, "key2": [5,6,7,8], "key3": ["arthur", "babar", "celeste"]}
+  |  +{
+  |  +    "key1": 1,
+  |  +    "key2": [
+  |  +        5,
+  |  +        6,
+  |  +        7,
+  |  +        8
+  |  +    ],
+  |  +    "key3": [
+  |  +        "arthur",
+  |  +        "babar",
+  |  +        "celeste"
+  |  +    ]
+  |  +}
+  |
+  o  changeset:   0:f168d21ce765
+     user:        test
+     date:        Thu Jan 01 00:00:00 1970 +0000
+     summary:     initial commit
+  
+     diff -r 000000000000 -r f168d21ce765 dir-1/file-1.json
+     --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+     +++ b/dir-1/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+     @@ -0,0 +1,1 @@
+     +{"key1": [42,53,78], "key2": [9,3,8,1], "key3": ["London", "Paris", "Tokyo"]}
+     diff -r 000000000000 -r f168d21ce765 dir-1/file-2.json
+     --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+     +++ b/dir-1/file-2.json	Thu Jan 01 00:00:00 1970 +0000
+     @@ -0,0 +1,1 @@
+     +{"key1": 1, "key2": [0, 1, 2, 3, 4, 5, 6, 7, 8, 9], "key3": [54]}
+     diff -r 000000000000 -r f168d21ce765 dir-2/file-1.json
+     --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+     +++ b/dir-2/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+     @@ -0,0 +1,1 @@
+     +{"key1": [44,50,82], "key2": [4, 8, -4, 5], "key3": ["Asia", "Europe"]}
+     diff -r 000000000000 -r f168d21ce765 dir-2/file-2.json
+     --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+     +++ b/dir-2/file-2.json	Thu Jan 01 00:00:00 1970 +0000
+     @@ -0,0 +1,3 @@
+     +{"key1": 6,
+     +"key2": [3, 5, -2, 3, 4, 11, 10, -4, 8, 9],
+     +"key3": [898, 32543, 2342]}
+     diff -r 000000000000 -r f168d21ce765 dir-2/file-3.json
+     --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+     +++ b/dir-2/file-3.json	Thu Jan 01 00:00:00 1970 +0000
+     @@ -0,0 +1,1 @@
+     +{"key1": "hello", "key2": [3, 1, 8, -1, 19, 2], "key3": "babar"}
+     diff -r 000000000000 -r f168d21ce765 root-file.json
+     --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+     +++ b/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+     @@ -0,0 +1,1 @@
+     +{"key1": 1, "key2": [5,6,7,8], "key3": ["arthur", "babar", "celeste"]}
+  
+
+Actual merge
+
+  $ hg diff -r 1 -r B_B .hg-format-source
+  $ hg diff -r 1 -r default .hg-format-source
+  diff -r 4d5d2129291f -r 73175e0546e6 .hg-format-source
+  --- a/.hg-format-source	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/.hg-format-source	Thu Jan 01 00:00:00 1970 +0000* (glob)
+  @@ -1,1 +1,4 @@
+   {"pattern": "glob:root-file.json", "tool": "json"}
+  +{"pattern": "glob:dir-1/**", "tool": "json"}
+  +{"pattern": "glob:dir-2/file-1.json", "tool": "json"}
+  +{"pattern": "glob:dir-2/file-3.json", "tool": "json"}
+  $ hg status
+  $ hg up -C B_B
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg merge default
+  merging dir-1/file-2.json
+  merging dir-2/file-3.json
+  merging root-file.json
+  3 files updated, 3 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg log -GT compact
+  @  10[tip]   db645e78c8a3   1970-01-01 00:00 +0000   test
+  |    update d2f2 and d2f3
+  |
+  o  9:5   b3816cb249a4   1970-01-01 00:00 +0000   test
+  |    update root-file and d1f2
+  |
+  | @  8   73175e0546e6   1970-01-01 00:00 +0000   test
+  | |    update d1f2 and d2f1
+  | |
+  | o  7   0daf45f1c8ad   1970-01-01 00:00 +0000   test
+  | |    update d1f1
+  | |
+  | o  6:3   180a5b3a4114   1970-01-01 00:00 +0000   test
+  | |    update root-file
+  | |
+  o |  5   654f4c32c118   1970-01-01 00:00 +0000   test
+  | |    conflicting update
+  | |
+  o |  4:1   217d6c23789b   1970-01-01 00:00 +0000   test
+  | |    update key 1
+  | |
+  | o  3   d29e366c2a4b   1970-01-01 00:00 +0000   test
+  | |    format some dir2 with explicite pattern
+  | |
+  | o  2   279459a606de   1970-01-01 00:00 +0000   test
+  |/     format dir1 as a whole
+  |
+  o  1   4d5d2129291f   1970-01-01 00:00 +0000   test
+  |    format the root-file
+  |
+  o  0   f168d21ce765   1970-01-01 00:00 +0000   test
+       initial commit
+  
+  $ hg diff -r 'p1()'
+  diff -r db645e78c8a3 .hg-format-source
+  --- a/.hg-format-source	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/.hg-format-source	* (glob)
+  @@ -1,1 +1,4 @@
+   {"pattern": "glob:root-file.json", "tool": "json"}
+  +{"pattern": "glob:dir-1/**", "tool": "json"}
+  +{"pattern": "glob:dir-2/file-1.json", "tool": "json"}
+  +{"pattern": "glob:dir-2/file-3.json", "tool": "json"}
+  diff -r db645e78c8a3 dir-1/file-1.json
+  --- a/dir-1/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/dir-1/file-1.json	* (glob)
+  @@ -1,1 +1,18 @@
+  -{"key1": [42,53,78], "key2": [9,3,8,1], "key3": ["London", "Paris", "Tokyo"]}
+  +{
+  +    "key1": [
+  +        42,
+  +        53,
+  +        76
+  +    ],
+  +    "key2": [
+  +        9,
+  +        3,
+  +        8,
+  +        1
+  +    ],
+  +    "key3": [
+  +        "London",
+  +        "Paris",
+  +        "Tokyo"
+  +    ]
+  +}
+  diff -r db645e78c8a3 dir-1/file-2.json
+  --- a/dir-1/file-2.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/dir-1/file-2.json	* (glob)
+  @@ -1,1 +1,18 @@
+  -{"key1": 4, "key2": [0, 1, 3, 3, 7, 5, 9, 7, 8, 9], "key3": [54]}
+  +{
+  +    "key1": 4,
+  +    "key2": [
+  +        0,
+  +        1,
+  +        3,
+  +        3,
+  +        7,
+  +        5,
+  +        9,
+  +        7,
+  +        8,
+  +        9
+  +    ],
+  +    "key3": [
+  +        54
+  +    ]
+  +}
+  diff -r db645e78c8a3 dir-2/file-1.json
+  --- a/dir-2/file-1.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/dir-2/file-1.json	* (glob)
+  @@ -1,1 +1,18 @@
+  -{"key1": [44,50,82], "key2": [4, 8, -4, 5], "key3": ["Asia", "Europe"]}
+  +{
+  +    "key1": [
+  +        44,
+  +        50,
+  +        86
+  +    ],
+  +    "key2": [
+  +        4,
+  +        8,
+  +        -4,
+  +        5
+  +    ],
+  +    "key3": [
+  +        "Asia",
+  +        "Europe",
+  +        "Oceania"
+  +    ]
+  +}
+  diff -r db645e78c8a3 dir-2/file-3.json
+  --- a/dir-2/file-3.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/dir-2/file-3.json	* (glob)
+  @@ -1,1 +1,12 @@
+  -{"key1": "hello", "key2": [6, 1, 8, -1, 19, 2], "key3": "Babar"}
+  +{
+  +    "key1": "hello",
+  +    "key2": [
+  +        6,
+  +        1,
+  +        8,
+  +        -1,
+  +        19,
+  +        2
+  +    ],
+  +    "key3": "Babar"
+  +}
+  diff -r db645e78c8a3 root-file.json
+  --- a/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/root-file.json	* (glob)
+  @@ -9,6 +9,7 @@
+       "key3": [
+           "arthur",
+           "babar",
+  -        "celeste"
+  +        "celeste",
+  +        "pomme"
+       ]
+   }
+  $ hg diff -r 'p2()'
+  diff -r 73175e0546e6 dir-1/file-2.json
+  --- a/dir-1/file-2.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/dir-1/file-2.json	* (glob)
+  @@ -3,11 +3,11 @@
+       "key2": [
+           0,
+           1,
+  -        2,
+  +        3,
+           3,
+  -        4,
+  +        7,
+           5,
+  -        6,
+  +        9,
+           7,
+           8,
+           9
+  diff -r 73175e0546e6 dir-2/file-2.json
+  --- a/dir-2/file-2.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/dir-2/file-2.json	* (glob)
+  @@ -1,3 +1,3 @@
+   {"key1": 6,
+   "key2": [3, 5, -2, 3, 4, 11, 10, -4, 8, 9],
+  -"key3": [898, 32543, 2342]}
+  +"key3": [898, 32543, 2336]}
+  diff -r 73175e0546e6 dir-2/file-3.json
+  --- a/dir-2/file-3.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/dir-2/file-3.json	* (glob)
+  @@ -1,12 +1,12 @@
+   {
+       "key1": "hello",
+       "key2": [
+  -        3,
+  +        6,
+           1,
+           8,
+           -1,
+           19,
+           2
+       ],
+  -    "key3": "babar"
+  +    "key3": "Babar"
+   }
+  diff -r 73175e0546e6 root-file.json
+  --- a/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/root-file.json	* (glob)
+  @@ -1,5 +1,5 @@
+   {
+  -    "key1": 1,
+  +    "key1": 1337,
+       "key2": [
+           5,
+           6,
+
+Merge from a sub directory
+==========================
+
+  $ hg up -C .
+  6 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ cd dir-1
+  $ hg merge default
+  merging dir-1/file-2.json
+  merging dir-2/file-3.json
+  merging root-file.json
+  3 files updated, 3 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg commit -qm 'Merge commit'
+  $ cd ..
+
+Preserve flags
+==============
+
+  $ mkdir flags
+  $ cat << EOF > flags/executable.json
+  > {}
+  > EOF
+  $ chmod +x flags/executable.json
+  $ hg add -q flags/*
+  $ hg ci -qm "Add flags subdir"
+  $ hg format-source json glob:flags/* -m 'Executable file formatted'
+  $ ls -l flags/* | cut -b 1-10
+  -rwxr-xr-x
+
+Test error case
+===============
+
+Current limitation
+------------------
+
+not at repository root
+
+  $ cd dir-1
+  $ hg format-source json glob:file-2.json
+  abort: format-source must be run from repository root
+  (cd $TESTTMP/test_repo)
+  [255]
+
+Not a glob:
+
+  $ cd ..
+  $ hg format-source json dir-1:file-2.json
+  abort: format-source only supports explicit 'glob' patterns for now ('dir-1:file-2.json')
+  (maybe try with "glob:dir-1:file-2.json")
+  [255]
+
+legitimate error
+----------------
+
+no pattern
+
+  $ hg format-source json
+  abort: no files specified
+  [255]
+
+unknown tool
+
+  $ hg format-source babar-tool glob:file-2.json
+  abort: unknow format tool: babar-tool (no 'format-source.babar-tool' config)
+  [255]
+
+space in tool
+
+  $ hg format-source 'babar tooling' 'glob:**'
+  abort: tool name cannot contains space: 'babar tooling'
+  [255]
+
+Uncommited changes
+
+  $ echo a >> root-file.json
+  $ hg format-source json 'glob:root-file.json'
+  abort: uncommitted changes
+  [255]
+
+Merge with an undefined tool
+
+  $ hg up -qC 0
+  $ cat << EOF > root-file.json
+  > {"key1": 1, "key2": [5,6,7,8], "key3": ["Arthur", "babar", "celeste"]}
+  > EOF
+  $ hg diff
+  diff -r f168d21ce765 root-file.json
+  --- a/root-file.json	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/root-file.json	* (glob)
+  @@ -1,1 +1,1 @@
+  -{"key1": 1, "key2": [5,6,7,8], "key3": ["arthur", "babar", "celeste"]}
+  +{"key1": 1, "key2": [5,6,7,8], "key3": ["Arthur", "babar", "celeste"]}
+  $ hg up --merge default --config 'format-source.json='
+  format-source, no command defined for 'json', skipping formating: 'root-file.json'
+  format-source, no command defined for 'json', skipping formating: 'root-file.json'
+  merging root-file.json
+  format-source, no command defined for 'json', skipping formating: 'root-file.json'
+  format-source, no command defined for 'json', skipping formating: 'root-file.json'
+  warning: conflicts while merging root-file.json! (edit, then use 'hg resolve --mark')
+  5 files updated, 0 files merged, 0 files removed, 1 files unresolved
+  use 'hg resolve' to retry unresolved file merges
+  [1]
diff --git a/tests/test-help.t b/tests/test-help.t
--- a/tests/test-help.t
+++ b/tests/test-help.t
@@ -360,6 +360,7 @@
        eol           automatically manage newlines in repository files
        extdiff       command to allow external programs to compare revisions
        factotum      http authentication with factotum
+       formatsource  help dealing with code source reformating
        githelp       try mapping git commands to Mercurial commands
        gpg           commands to sign and verify changesets
        hgk           browse the repository in a graphical way
diff --git a/tests/testlib/json-pretty.py b/tests/testlib/json-pretty.py
new file mode 100644
--- /dev/null
+++ b/tests/testlib/json-pretty.py
@@ -0,0 +1,47 @@
+r"""Command-line tool to validate and pretty-print JSON
+
+Usage::
+
+    $ echo '{"json":"obj"}' | python -m json.tool
+    {
+        "json": "obj"
+    }
+    $ echo '{ 1.2:3.4}' | python -m json.tool
+    Expecting property name enclosed in double quotes: line 1 column 3 (char 2)
+
+"""
+from __future__ import absolute_import
+import json
+import os
+import sys
+
+def main():
+    if len(sys.argv) == 1:
+        infile = sys.stdin
+        outfile = sys.stdout
+    elif len(sys.argv) == 2:
+        infile = open(sys.argv[1], 'rb')
+        outfile = sys.stdout
+    elif len(sys.argv) == 3:
+        infile = open(sys.argv[1], 'rb')
+        outfile = open(sys.argv[2], 'wb')
+    else:
+        raise SystemExit(sys.argv[0] + " [infile [outfile]]")
+    with infile:
+        try:
+            obj = json.load(infile)
+        except ValueError as e:
+            raise SystemExit(e)
+    if os.path.exists('.json-indent'):
+        with open('.json-indent') as fp:
+            indent = int(fp.read())
+    else:
+        indent = 4
+    with outfile:
+        json.dump(obj, outfile, sort_keys=True,
+                  indent=indent, separators=(',', ': '))
+        outfile.write('\n')
+
+
+if __name__ == '__main__':
+    main()