Update merge request's merge_commit after branch update

What does this MR do?

When a branch gets new commits, some of those might belong to merge request which are still open. We currently close those MRs, but they will not have merge_commit set.

An analyzer is implemented, to analyze commits related to the new change, and determine each commit's merge commit, relative to the branch.

What are the relevant issue numbers?

Closes #48889 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• 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.