Support merge ref writing (without merging to target branch)

This is the Gitaly counterpart for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24692 and gitaly-proto!261 (merged).

It adds a new RPC which allows a merge between a source SHA and a target branch, but instead changing the target branch, we're aiming to write it in a target_ref. Mainly, we're looking forward to use the refs/merge-requests/:iid/merge namespace.

In order to make the RPC idempotent, we always trash the given target_ref before writing the commit on it.