diff --git a/mercurial/cmdutil.py b/mercurial/cmdutil.py
--- a/mercurial/cmdutil.py
+++ b/mercurial/cmdutil.py
@@ -755,7 +755,8 @@ class changeset_templater(changeset_prin
                                      cache={
                                          'parent': '{rev}:{node|formatnode} ',
                                          'manifest': '{rev}:{node|formatnode}',
-                                         'filecopy': '{name} ({source})'})
+                                         'filecopy': '{name} ({source})',
+                                         'extra': '{key}={value|stringescape}'})
         # Cache mapping from rev to a tuple with tag date, tag
         # distance and tag name
         self._latesttagcache = {-1: (0, 0, 'null')}
diff --git a/tests/test-command-template b/tests/test-command-template
--- a/tests/test-command-template
+++ b/tests/test-command-template
@@ -93,7 +93,7 @@ cat changelog
 
 echo "# keys work"
 for key in author branches date desc file_adds file_dels file_mods \
-        files manifest node parents rev tags diffstat; do
+        files manifest node parents rev tags diffstat extras; do
     for mode in '' --verbose --debug; do
         hg log $mode --template "$key$mode: {$key}\n"
     done
diff --git a/tests/test-command-template.out b/tests/test-command-template.out
--- a/tests/test-command-template.out
+++ b/tests/test-command-template.out
@@ -569,6 +569,33 @@ diffstat--debug: 1: +1/-0
 diffstat--debug: 1: +4/-0
 diffstat--debug: 1: +2/-0
 diffstat--debug: 1: +1/-0
+extras: branch=default
+extras: branch=default
+extras: branch=default
+extras: branch=default
+extras: branch=foo
+extras: branch=default
+extras: branch=default
+extras: branch=default
+extras: branch=default
+extras--verbose: branch=default
+extras--verbose: branch=default
+extras--verbose: branch=default
+extras--verbose: branch=default
+extras--verbose: branch=foo
+extras--verbose: branch=default
+extras--verbose: branch=default
+extras--verbose: branch=default
+extras--verbose: branch=default
+extras--debug: branch=default
+extras--debug: branch=default
+extras--debug: branch=default
+extras--debug: branch=default
+extras--debug: branch=foo
+extras--debug: branch=default
+extras--debug: branch=default
+extras--debug: branch=default
+extras--debug: branch=default
 # filters work
 
 hostname