Support 'git grep <match> <branch>' type searches in gclient grep.
These searches had a slightly different output so needed more
tweeking to the regex.  I cleaned it up a bit too.

R=maruel@chromium.org
BUG=


Review URL: https://chromiumcodereview.appspot.com/11316213

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@170039 0039d316-1c4b-4281-b951-d872f2087c98