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

hgext/remotefilelog/fileserverclient.py

@@ -544,7 +544,7 b' class fileserverclient(object):'
                     if fetchwarning:
                         self.ui.warn(fetchwarning + '\n')
                 self.logstacktrace()
-            missingids = [(file, hex(id)) for file, id in missingids]
+            missingids = [(file, hex(id)) for file, id in sorted(missingids)]
             fetched += len(missingids)
             start = time.time()
             missingids = self.request(missingids)