WIP: Merge Request commits not longer in the branch stay in discussion notes

What does this MR do?

Outdated commits (commits that were previously in a merge request and now are not) might still have discussions on them. This PR keeps those discussions under the discussions tab of a MR.

Are there points in the code the reviewer needs to double check?

• Efficiency of commit notes lookup

Why was this MR needed?

• Merge Request history conversation history is important. Losing notes might lose some context about why a decision was made.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #24323 (closed)