diff --git a/mercurial/cmdutil.py b/mercurial/cmdutil.py
--- a/mercurial/cmdutil.py
+++ b/mercurial/cmdutil.py
@@ -170,7 +170,7 @@
 
 diffopts = [
     (b'a', b'text', None, _(b'treat all files as text')),
-    (b'g', b'git', None, _(b'use git extended diff format')),
+    (b'g', b'git', None, _(b'use git extended diff format (DEFAULT: diff.git)')),
     (b'', b'binary', None, _(b'generate binary diffs in git mode (default)')),
     (b'', b'nodates', None, _(b'omit dates from diff headers')),
 ]
diff --git a/mercurial/help.py b/mercurial/help.py
--- a/mercurial/help.py
+++ b/mercurial/help.py
@@ -152,8 +152,17 @@
     doc = b''.join(rst)
     return doc
 
+def parsedefaultmarker(text):
+    """given a text 'abc (DEFAULT: def.ghi)',
+    returns (b'abc', (b'def', b'ghi')). Otherwise return None"""
+    if text[-1:] == b')':
+        marker = b' (DEFAULT: '
+        pos = text.find(marker)
+        if pos >= 0:
+            item = text[pos + len(marker):-1]
+            return text[:pos], item.split(b'.', 2)
 
-def optrst(header, options, verbose):
+def optrst(header, options, verbose, ui):
     data = []
     multioccur = False
     for option in options:
@@ -165,7 +174,14 @@
 
         if not verbose and any(w in desc for w in _exclkeywords):
             continue
-
+        defaultstrsuffix = b''
+        if default is None:
+            parseresult = parsedefaultmarker(desc)
+            if parseresult is not None:
+                (desc, (section, name)) = parseresult
+                if ui.configbool(section, name):
+                    default = True
+                    defaultstrsuffix = _(b' from config')
         so = b''
         if shortopt:
             so = b'-' + shortopt
@@ -183,7 +199,7 @@
             defaultstr = pycompat.bytestr(default)
             if default is True:
                 defaultstr = _(b"on")
-            desc += _(b" (default: %s)") % defaultstr
+            desc += _(b" (default: %s)") % (defaultstr + defaultstrsuffix)
 
         if isinstance(default, list):
             lo += b" %s [+]" % optlabel
@@ -714,11 +730,11 @@
 
         # options
         if not ui.quiet and entry[1]:
-            rst.append(optrst(_(b"options"), entry[1], ui.verbose))
+            rst.append(optrst(_(b"options"), entry[1], ui.verbose, ui))
 
         if ui.verbose:
             rst.append(
-                optrst(_(b"global options"), commands.globalopts, ui.verbose)
+                optrst(_(b"global options"), commands.globalopts, ui.verbose, ui)
             )
 
         if not ui.verbose:
@@ -858,7 +874,7 @@
         elif ui.verbose:
             rst.append(
                 b'\n%s\n'
-                % optrst(_(b"global options"), commands.globalopts, ui.verbose)
+                % optrst(_(b"global options"), commands.globalopts, ui.verbose, ui)
             )
             if name == b'shortlist':
                 rst.append(
diff --git a/tests/test-help.t b/tests/test-help.t
--- a/tests/test-help.t
+++ b/tests/test-help.t
@@ -788,6 +788,12 @@
   (use 'hg help extensions' for information on enabling extensions)
   [255]
 
+Checking that help adapts based on the config:
+
+  $ hg help diff --config ui.tweakdefaults=true | egrep -e '^ *(-g|config)'
+   -g --[no-]git            use git extended diff format (default: on from
+                            config)
+
 Make sure that we don't run afoul of the help system thinking that
 this is a section and erroring out weirdly.