Add support first_parent_ref in UserMergeToRef

Problem to solve

We need to generate a merge ref from the other merge ref (instead of a target branch) in order to run pipelines for merge trains in parallel. This is a necessary component to accomplish our final goal Merge train feature.

• Issue: https://gitlab.com/gitlab-org/gitlab-ee/issues/11222
• Rails: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14298
• Gitaly proto: gitaly-proto!314 (merged)