Fix git cl comments when there's no issue associated with the current branch.

It currently crashes, which is sad.

R=pgervais@chromium.org
BUG=

Review URL: https://codereview.chromium.org/683283006

git-svn-id: http://src.chromium.org/chrome/trunk/tools/depot_tools@292899 4ff67af0-8c30-449e-8e8b-ad334ec8d88c