If the ref hasn't been fetched into the target repository yet, this will fail
with a Rugged::ReferencError (assuming we're not using Gitaly). We should handle
this in the same way as a missing ref.
This does two things:
- Pass commit oids instead of `Gitlab::Git::Commit`s. We only need the
former.
- Depend on only the target repository for conflict listing. For
conflict resolution, treat one repository as a remote one so that we can
implement it as such in Gitaly.
