Add merged_commit_sha to queries for merge requests

What does this MR do and why?

This column was recently added, but not to any queries because the index did not exist yet. This index has now been added, so we can use it in queries. This means that non-squashed fast-forward merge train merges can now be traced back to the source MR.

See #425100 (closed)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Database review

Updated queries with postgres.ai links have been attached to the corresponding code changes.