On GitLab.com, 19M of 93M rows have NULL external_diff_store.

With batches of 5000 and a background migration job interval of 2m 5s,
3.8K jobs are scheduled over 5.5 days.

The index `index_merge_request_diffs_external_diff_store_is_null` is
expected to be used by the migration queries.

queue_background_migration_jobs_by_range_at_intervals is not used
because it would enqueue 18.6K jobs and we have an index for getting
these ranges.