Draft: feat(code-review): Rebuild missing diff commits from the repository

What does this MR do and why?

feat(code-review): Rebuild missing diff commits from the repository

Diffs whose commit rows were missed in the migration to the partitioned table report zero commits. When a diff has a non-zero commits_count but no rows, rebuild the list from the repository instead of returning empty

Read-path only and behind mr_diff_commits_repository_fallback, until the rows are backfilled.

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