This includes steps 2-4 of T215466:
- 2. Introduce a migration from using revision_comment_temp and revision_actor_temp (old) to rev_comment_id and rev_actor (new).
- 2.1. Write and merge the MediaWiki patch, assuming it wasn't included in 1.1.
- 2.2. Check for extensions assuming the temp tables exist.
- 3. Set migration stage to write-both/read-old.
- 4. Run the maintenance script at WMF to backfill rev_comment_id and rev_actor.
- Add the migration script to the MW installer/updater.
Keeping in mind: