diff --git a/mercurial/mdiff.py b/mercurial/mdiff.py
index 0ae287eb6a4fee5b7aa685e356dbac5f8187c3ef_bWVyY3VyaWFsL21kaWZmLnB5..e2796f193f067cc87fe34dec5fb2d5a1b13e78d2_bWVyY3VyaWFsL21kaWZmLnB5 100644
--- a/mercurial/mdiff.py
+++ b/mercurial/mdiff.py
@@ -57,6 +57,7 @@
         'ignorewsamount': False,
         'ignoreblanklines': False,
         'upgrade': False,
+        'showsimilarity': False,
         }
 
     def __init__(self, **opts):
diff --git a/mercurial/patch.py b/mercurial/patch.py
index 0ae287eb6a4fee5b7aa685e356dbac5f8187c3ef_bWVyY3VyaWFsL3BhdGNoLnB5..e2796f193f067cc87fe34dec5fb2d5a1b13e78d2_bWVyY3VyaWFsL3BhdGNoLnB5 100644
--- a/mercurial/patch.py
+++ b/mercurial/patch.py
@@ -2169,6 +2169,11 @@
     if git:
         buildopts['git'] = get('git')
 
+        # since this is in the experimental section, we need to call
+        # ui.configbool directory
+        buildopts['showsimilarity'] = ui.configbool('experimental',
+                                                    'extendedheader.similarity')
+
         # need to inspect the ui object instead of using get() since we want to
         # test for an int
         hconf = ui.config('experimental', 'extendedheader.index')