Reschedule commits_count background migration
We still have >100K unmigrated MergeRequestDiffs which don't have commits_count set yet (see https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17567#note_61904891)
This migration re-schedules the original background migration. To assure that records are not processed twice, records with commits_count set are skipped.
Related to #41698 (closed) and !17567 (merged)