Summary: Fixes T13457. Ref T13444. When we iterate over commits in a particular repository, the default iteration strategy can't effectively use the keys on the table. Tweak the ordering so the "<repositoryID, epoch, [id]>" key can be used. Test Plan: - Ran `bin/audit delete --repository X` and `bin/repository rebuild-identities --repository X` before and after changes. - With just the key changes, performance was slightly better. My local data isn't large enough to really emphasize the key changes. - With the page size changes, performance was a bit better (~30%, but on 1-3 second run durations). - Used `--trace` and ran `EXPLAIN ...` on the new queries, saw them select the "<repositoryID, epoch, [id]>" key and report a bare "Using index condition" in the "Extra" column. Maniphest Tasks: T13457, T13444 Differential Revision: https://secure.phabricator.com/D20921