Validate results of MR diffs and commits background migration

Same as https://gitlab.com/gitlab-com/infrastructure/issues/2663, but hopefully successful.

I need to see the output of:

SELECT COUNT(*) FROM merge_request_diffs WHERE st_commits IS NOT NULL OR st_diffs IS NOT NULL;

If that is non-zero, then I'll need the results of:

SELECT id FROM merge_request_diffs WHERE st_commits IS NOT NULL OR st_diffs IS NOT NULL;

And more, as @victorcete helped me with last time: https://gitlab.com/gitlab-com/infrastructure/issues/2663#note_39244715