R1:b3976acc40a6

Improve performance of "phabricator:20210215.changeset.02.phid-populate.php"

Summary: Ref T13613. Improve the performance of this migration by using a temporary table and an "UPDATE x JOIN y ..." pattern.

Test Plan:
- Ran on `secure`, got exit after a few seconds since the migration is idempotent and changesets already had PHIDs.
- Ran on `secure` with the `continue;` commented out, got valid new PHIDs in 53s (from 153s).
- Tried a larger page size (16K), didn't see any improvement.
- From "…