Add legacy columns to partitioned commits table

Eugenia Grieffrequested to merge
527241-add-commits-columns-to-partitoned into master

What does this MR do and why?

Add legacy columns to partitioned commits table

Adds the seven columns that exist on merge_request_diff_commits but not on merge_request_diff_commits_b5377a7a34, as always-NULL compatibility columns.

The swap renames the partitioned table over the legacy one. Code paths guarded by mr_diff_commits_read_new_table being disabled still name sha, trailers, commit_author_id and the rest, so without these columns those queries raise PG::UndefinedColumn between the swap and the flag reaching 100%. Nothing writes them: create_bulk strips DEDUPLICATED_COLUMNS and trailers before insert.

References

https://gitlab.com/gitlab-org/gitlab/-/work_items/602841

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Related to #527241

Edited by Eugenia Grieff

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