When using `git-webkit pr`, if a user has a local commit on a production branch that is not pushed to the remote, we could simply move the local ref for the production branch to wherever the remote ref is and use the local commit as the change to be uploaded. Right now, we raise an error because the PR branch doesn't have any differences to it's target branch.
<rdar://problem/88346615>
Landed 247151@main (r289659)