Create an user facing API to write / refresh the MR merge ref
This is a follow-up for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24692.
There we create the groundwork which involves using a new RPC to accomplish that task (gitaly!1057 (merged)) through a separate MergeToRefService
.
This issue will solve the usability part, where we provide an API to actually call MergeToRefService
make it useful to external users / services.