Remove almost all columns from merge_request_diff_commits

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Originally, merge_request_diff_commits was meant to have three columns:

1. merge_request_diff_id
2. relative_order
3. sha

However, for simplicity, I added columns representing almost all of the fields we serialised in st_commits. The only exception was parent_ids, as it appeared to be unused.

We should try to find which other columns are unused and remove those, until we're left with the original three. Commit metadata for an MR diff can come from its Compare object, with a fallback if there are old MR diffs without commits kept around in the repo.

This is currently blocked by https://gitlab.com/gitlab-org/gitlab-ce/issues/34743.