Handle a partially-updated RemoteMirror

In gitaly!1915 (merged), we avoided pushing refs to a remote mirror that had diverged from the local source (assuming keep_divergent_refs was enabled for the call).

This had the side effect of making it look like a mirror was successful, even if some refs had diverged and weren't even attempted to be pushed.

In order to prevent a "false positive" of a fully-updated mirror, we need to inform the user that some refs weren't pushed at all.

This merge request consumes the new divergent_refs attribute of the updated UpdateRemoteMirrorResponse protocol, which contains a list of refs that were diverged and thus not pushed. See gitaly#2557 (closed).

Gitaly counterpart: gitaly!2028 (merged).