Let `git cl format` format .mm files too.

(It's a bit unfortunate that we're duplicating the default -regex list in
clang-format-diff.py here :-/ But the .js and .proto stuff is probably not
quite ready yet anyhow, so I'm only adding .mm for now.)

TEST=Edit a .mm file, commit, run `git cl format`, `git diff` shows formatted
output.
TBR=iannucci@chromium.org

Review URL: https://codereview.chromium.org/152303002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@248424 0039d316-1c4b-4281-b951-d872f2087c98

git_cl.py

@@ -2306,7 +2306,7 @@ def CMDowners(parser, args):
 @subcommand.usage('[files or directories to diff]')
 def CMDformat(parser, args):
   """Runs clang-format on the diff."""
-  CLANG_EXTS = ['.cc', '.cpp', '.h']
+  CLANG_EXTS = ['.cc', '.cpp', '.h', '.mm']
   parser.add_option('--full', action='store_true',
                     help='Reformat the full content of all touched files')
   parser.add_option('--dry-run', action='store_true',