Support merge ref writing (without merging to target branch)

What does this MR do?

Adds the ground work for writing the merge ref to refs/merge-requests/:iid/merge, merging the source and target branches of a MR without changing the target branch.

EE MR https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9471

• Solve failed to create commit: current tip is not the first parent after first merge to ref is made
• Improve tests
• Get a green pipeline

Follow ups: https://gitlab.com/gitlab-org/gitlab-ce/issues/57920, https://gitlab.com/gitlab-org/gitlab-ce/issues/57252

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/47110

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer