Otherwise, I get a stack trace, which is annoying. Here's an example:

$ git cl upload
Current branch help_text is up to date.

Failed to diff against upstream branch refs/heads/foo!

This branch probably doesn't exist anymore. To reset the
tracking branch, please run
    git branch --set-upstream help_text trunk
replacing trunk with origin/master or the relevant branch


R=dpranke@chromium.org
BUG=
TEST=


Review URL: http://codereview.chromium.org/8349011

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@106175 0039d316-1c4b-4281-b951-d872f2087c98