The parsing of change descriptions had a lot of overlap and inconsistencies between gcl and git-cl. In particular, we weren't handling TBR= consistently, or probably a few other things.

This change moves most of the code into presubmit_support and gclient_utils and just leaves the formatting differences for the messages between the two tools.

Review URL: http://codereview.chromium.org/6719004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@79002 0039d316-1c4b-4281-b951-d872f2087c98