Batch up SQL operations in the `./bin/repository parents` script.

Summary: Fixes T5255. Currently the `./bin/repository parents` workflow is quite slow. Batching up the SQL operations should make the workflow //seem// much faster.

Test Plan: Not yet tested.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5255

Differential Revision: https://secure.phabricator.com/D9361