remotefilelog: prefetch files in deterministic order
I have been troubleshooting some slowness in this area (it's unclear
if it's the client or server that's to blame, but that's beside the
point) and it's a lot easier to do troubleshoot if the files are
prefetched in the same order each time.
Differential Revision: https://phab.mercurial-scm.org/D6172