largefiles: batch statlfile requests when pushing a largefiles repo (issue3386) This implements a part of issue 3386. It batches the request for the status of all largefiles in the revisions that are about to be pushed into a single request, instead of doing N separate requests. In a real world test case, this change was verified to save 1,116 round-trips to the server. It only requires a client-side change; it is backwards-compatible with an older version of the server.