Add UserRebaseToRef RPC
Split out of !5897 (closed) as discussed in !5897 (comment 1438633612).
This adds a new RPC, analogous to UserMergeToRef, that allows
constructing refs/merge-requests/
references via a rebase as opposed
to a merge. This is part of enabling merge trains in projects that
require fast-forward merges.
Edited by Hordur Freyr Yngvason