Merge MR by updating target branch to point to merge ref HEAD
Once we do https://gitlab.com/gitlab-org/gitlab-ce/issues/58495 and https://gitlab.com/gitlab-org/gitlab-ce/issues/58226, we can trust the merge ref to always be up to date if merge_status == can_be_merged, we should be able to reimplement the MergeRequests::MergeService as MergeRequests::UpdateMergeRefService.new(...).execute(...) && repository.write_ref(target_branch, merge_ref). This means that, assuming the merge ref is already up to date, performing the merge should be near-instantaneous.
Of course, we will still need to verify that the target branch HEAD that was used to compute the merge ref actually matches what we're about to overwrite. We could return this SHA from the MergeToRefService that will likely be called from inside the UpdateMergeRefService, but that means it wouldn't be available if the merge ref is already up to date by the time the MergeToRefService is called. We may need to store the source and target branch HEADs that were used to compute the merge ref somewhere so we have access to it at this time.
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