gclient grep: support pathspec arguments
gclient grep: support pathspec arguments

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: bd64dd6fa6f3f0ed0c0666d1018379882b742947@170039 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

svn2github/chromium-depot-tools,svn2github/chromium-depot-tools,svn2github/chromium-depot-tools