Support fast forward methods for merge ref operation

Problem to solve

If I checkout the merge ref for a merge request refs/merge-requests/$iid/merge for a project that uses a fast forward or semi-linear merge strategy, the ref is not representative of the the change because it will actually contain commits that cannot be merged in a fast forward arrangement with master.

This means that the merge-ref isn't an accurate representation of the resultant state of the target branch should the merge occur.

Further details

Follow up to https://gitlab.com/gitlab-org/gitlab-ce/issues/47110

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24692 introduces a MergeToRefService which writes to refs/merge-requests/:iid/merge, though it currently only supports the merge method, which currently writes a new commit SHA to the ref. Ideally we should also support squash and FF-only projects / MRs.

Proposal

1. This will probably require a new RPC UserFfToRef (as opposed to UserMergeToRef we currently have).
2. Change MergeToRefService to use it instead raising an "unsupported" error

Links / references