Validate results of background migration

https://gitlab.com/gitlab-org/gitlab-ce/issues/36631 is an issue that didn't reappear since we changed the DB setup.

To validate that it's not needed - and that the background migration is fully complete - we should run this:

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 probably some more information, too 🙂