Directly write SHA of a merge train ref to the target branch instead of creating a new commit

Summary

Currently, Merge Trains discard temporary refs (cascading/combined refs) after a merge request is merged, however, we can use this temporary ref more effectively by directly writing the SHA into the target branch ref.

Details

When a merge train runs, here is what happens in general:

1. An MR gets on a merge train.
2. The MR creates a cascading ref as a SHA SHA-TMP-1.
3. The MR creates a new pipeline on SHA-TMP-1, and this pipeline has passed.
4. The MR gets merged, a merge train creates a new SHA SHA-1 on the target branch.

The point of this issue is to replace the SHA-1 by SHA-TMP-1 when the target branch is updated. Technically, these SHA contains the same history so that end result will be identical.

This issue is similar to #27117.

Use Cases

1. If a commit SHA in an artifact used for identification later (what build is running?) then projects need to rebuild to get the real sha

Concerns

• What if the merge method is not no-ff? (e.g. ff-merge-only, squash option, etc)

Proposal

Directly write SHA of a merge train ref to the target branch instead of creating a new commit.

Reference

• #35628 (comment 289655931)