diff --git a/mercurial/templatefilters.py b/mercurial/templatefilters.py
--- a/mercurial/templatefilters.py
+++ b/mercurial/templatefilters.py
@@ -7,6 +7,7 @@
 
 from __future__ import absolute_import
 
+import functools
 import os
 import re
 import time
@@ -99,6 +100,38 @@
     """
     return os.path.basename(path)
 
+@templatefilter('commonprefix')
+def commonprefix(filelist):
+    """List of text. Treats each list item as file name with /
+    as path separator and returns the longest common directory
+    prefix shared by all list items.
+    Returns the empty string if no common prefix exists.
+
+    The list items are not normalized, i.e. "foo/../bar" is handled as
+    file "bar" in the directory "foo/..". Leading slashes are ignored.
+
+    For example, ["foo/bar/baz", "foo/baz/bar"] becomes "foo" and
+    ["foo/bar", "baz"] becomes "".
+    """
+    seen = set()
+    def common(a, b):
+        if b in seen:
+            return a
+        seen.add(b)
+        if len(a) > len(b):
+            a, b = b, a
+        for i in xrange(len(a)):
+            if a[i] != b[i]:
+                return a[:i]
+        return a
+    try:
+        if not filelist:
+            return ""
+        dirlist = [tuple(f.lstrip('/').split('/')[:-1]) for f in filelist]
+        return '/'.join(functools.reduce(common, dirlist))
+    except TypeError:
+        raise error.ParseError(_('argument is not a list of text'))
+
 @templatefilter('count')
 def count(i):
     """List or text. Returns the length as an integer."""
diff --git a/tests/test-template-filters.t b/tests/test-template-filters.t
new file mode 100644
--- /dev/null
+++ b/tests/test-template-filters.t
@@ -0,0 +1,21 @@
+  $ hg debugtemplate '{""|splitlines|commonprefix}\n'
+  
+  $ hg debugtemplate '{"foo/bar\nfoo/baz\n"|splitlines|commonprefix}\n'
+  foo
+  $ hg debugtemplate '{"foo/bar\nfoo/bar\n"|splitlines|commonprefix}\n'
+  foo
+  $ hg debugtemplate '{"/foo/bar\n/foo/bar\n"|splitlines|commonprefix}\n'
+  foo
+  $ hg debugtemplate '{"/foo\n/foo\n"|splitlines|commonprefix}\n'
+  
+  $ hg debugtemplate '{"foo/bar\nbar/baz"|splitlines|commonprefix}\n'
+  
+  $ hg debugtemplate '{"foo/../bar\nfoo/bar"|splitlines|commonprefix}\n'
+  foo
+  $ hg debugtemplate '{"foo\n/foo"|splitlines|commonprefix}\n'
+  
+  $ hg init
+  $ hg log -r null -T '{rev|commonprefix}'
+  hg: parse error: argument is not a list of text
+  (template filter 'commonprefix' is not compatible with keyword 'rev')
+  [255]