Turn "bypassCache" into a no-op in "diffusion.querycommits"

Summary: Ref T13552. The internal caller for this now uses "internal.commit.search", which is always authority-reading. No legitimate external caller should rely on the behavior of "bypassCache"; no-op it to simplify behavior.

Test Plan: Called "diffusion.querycommits", saw the same data as before.

Maniphest Tasks: T13552

Differential Revision: https://secure.phabricator.com/D21447