R1:63d84e0b44b7

Improve use of keys when iterating over commits in "bin/audit delete" and "bin/repository rebuild-identities"

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…