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 to backfill `rev_comment_id` and `rev_actor`.
1) Write to rev_actor and rev_comment_id
2) Backfill rev_actor and rev_comment_id
3) have a read-new mode, at least for testing
* We won’t get through with the migration before the 1.36 branch (March?)
** We should avoid supporting multiple modes in a release
* But we can start writing to the new schema, and we can also already backfill.
* The Updater to 1.36 should also backfill.