Add merged_commit_sha to merge_requests

What does this MR do and why?

This adds a new column for tracking any type of merge back to a merge request. This is important because an automatic rebase during merge would create a commit SHA that does not exist on the source branch. This will be used when fast-forward merging merge trains from the merge train ref, where the merge train ref has been constructed via a sequence of squashes, rebases and merge commits.

See #418822 (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

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