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.