improvements.

  * No longer crash on HEAD
  * Always correctly highlight lines based on hash-matching-ness
  * Only import git_cl if you want it (it can be slow)
  * Print a message when NO branches are found (instead of blank output)

R=calamity@chromium.org, jsbell@chromium.org
BUG=410353

Review URL: https://codereview.chromium.org/554523003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@291847 0039d316-1c4b-4281-b951-d872f2087c98