Draft: Do not merge: DB testing scratch for !255692

What does this MR do and why?

fix(code-review): Backfill diff commits missed by partitioning

The original backfill excluded rows with a NULL sha, commit_author_id or committer_id. That is the shape of every row written once merge_request_diff_commits_dedup was enabled, so those rows were never copied into the partitioned table.

Queues a GitLab.com-only backfill bounded to the cursor range between the dedup flag being enabled and the partition sync trigger taking over, so the archived table is not rescanned in full.

https://gitlab.com/gitlab-org/gitlab/-/work_items/628194

Changelog: other

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Related to #628194