Don't push divergent remote branches with keep_divergent_refs

Previously, if a remote branch had completely diverged from the source (i.e., it is no longer an ancestor of the source), we'd still attempt to push to it and would only realize it failed when the server came back with an error.

Now when selecting local branches to push to the remote, the keep_divergent_refs option allows us to limit the list to only branches that have no divergence.

This is an alternative implementation to !1899 (closed) suggested (and largely implemented) by @jacobvosmaer-gitlab.