Introduce a new schema migration stage in which rev_actor is used
directly and the revision_actor_temp table is no longer needed. This
becomes the new "new" stage whereas the previous situation is now
termed SCHEMA_COMPAT_TEMP.
Introduce migrateRevisionActorTemp which copies data from
revision_actor_temp to rev_actor. The code is similar to
migrateImageCommentTemp.php except that it doesn't delete from the old
table.
Partial revert of c29909e59f. That change removed direct
references to $wgActorTableSchemaMigrationStage and made queries
involving revision_actor_temp be unconditional. Such changes need to be
reverted to make the use of revision_actor_temp be conditional again.
In ActorMigrationTest, I compacted provideGetJoin() and
provideGetWhere(), removing most of the duplication between expected
values. I gave all the stages a short name, and mostly used the name in
providers.
Bug: T275246
Change-Id: I7498107dd6433ab7de5bf2e7b3fe2aa5e10e345d
