patch: add label for coloring the similarity extended header

Just like the summary says, this will colorize the:

  similarity index 88%

line in the diff output.

mercurial/patch.py

@@ -2350,6 +2350,7 b' def difflabel(func, *args, **kw):'
                     ('new', 'diff.extended'),
                     ('deleted', 'diff.extended'),
                     ('index', 'diff.extended'),
+                    ('similarity', 'diff.extended'),
                     ('---', 'diff.file_a'),
                     ('+++', 'diff.file_b')]
     textprefixes = [('@', 'diff.hunk'),