Fork divergence

@rpaik

I have my personal fork of GitLab by accident I pushed into my master branch, causing it to diverge and it can no longer automatically sync via mirroring.

I think the following should be available:

1. If the fork uses automatic mirroring it obviously means you want to use the origin master. Hence any push on master of the fork requires some confirmation with proper alert that it will cause divergence.

2. If the branch diverged, we should have an easy way to fix it via the UI.