patch: add similarity config knob in experimental section

This config knob will control whether or not to show the similarity
calculation in the diff output:

  diff --git a/README.md b/foo.md
  similarity index 88%
  rename from README.md
  rename to foo.md
  --- a/README.md
  +++ b/foo.md

mercurial/mdiff.py

@@ -57,6 +57,7 b' class diffopts(object):'
         'ignorewsamount': False,
         'ignoreblanklines': False,
         'upgrade': False,
+        'showsimilarity': False,
         }
 
     def __init__(self, **opts):

mercurial/patch.py

@@ -2169,6 +2169,11 b' def difffeatureopts(ui, opts=None, untru'
     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')