Optimize commit storage strategy

What does this MR do and why?

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/517497

Experiment: skip commit message persistence in the database, because this data is loaded directly from Gitaly.

References

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• https://gitlab.com/gitlab-org/gitlab/-/issues/517497

MR acceptance checklist

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Before	After

How to set up and validate locally

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

1. In rails console enable the experiment fully

   Feature.enable(:optimized_commit_storage)

2. Create a merge request and push some commits into it
3. Check the state of message column for latest MergeRequestDiff

   merge_request.latest_merge_request_diff.merge_request_diff_commits

4. Message column value for created commits should be empty.

Related to #517497