Track merge train ref merge params

What does this MR do and why?

We are in the progress of implementing merge train merges as fast-forward merges directly from the merge train ref. At the time of merge, we need to update the merge_commit_sha and squash_commit_sha attributes of the merge request. Since the preferences used to create the ref may have changed in the meantime, we need to persist the necessary values when creating the ref.

Part of #422483 (closed)

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