Add a fast path to avoid checking every commit on push

If a user can forge author, commiter and gerrit server identity, and
can upload merges, don't bother checking the commit history of what is
being uploaded. This can save time on servers that are trying to
accept a large project import using the push permission.

Change-Id: Idfc49c5fff93d51f38297f25b8757a1ce2b8564a