Add support for two-step Gitaly Rebase RPC

The new two-step Gitaly `Rebase` RPC yields the rebase commit SHA to the
client before proceeding with the rebase.

This avoids an issue where the rebase commit SHA was returned when the
RPC had fully completed, and in some cases this would be after the Rails
`post_receive` worker services had already run. In these situations,
the merge request did not yet have its rebase_commit_sha attribute set
introducing the possibility for bugs (such as previous approvals being
reset).

https://gitlab.com/gitlab-org/gitlab-ee/issues/5966