Make 'git cl patch' work when not on a named branch.

R=maruel@chromium.org,pgervais@chromium.org
BUG=

Review URL: https://codereview.chromium.org/661413005

git-svn-id: http://src.chromium.org/chrome/trunk/tools/depot_tools@292537 4ff67af0-8c30-449e-8e8b-ad334ec8d88c