This is not only problem with `differential.querydiffs` itself.
"local:commits" are used in arcanist to generate new comment when adding more changes via `arc diff` on the same revision. If "local:commits" is missing, arcanist takes all commits (except first one) as "new" and includes them all into the message on revision.
**Reproduction:**
- Create a new diff: `arc diff`
- Change something, update diff `arc diff`
- Change something, update diff `arc diff` (yes, second time)
- All local commits will appear on diff comment message.