Fix "git cl diff" when run on branch without an issue.

BUG=321279

Review URL: https://codereview.chromium.org/86133002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@237110 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed