discovery: limit 'all local heads known remotely' to real 'all' (issue4438)

3ef893520a85 made it possible that the initial head check didn't include all
heads. If that is the case, don't use the early exit just because this random
sample happened to be 'all known'.

Note: the randomness in the discovery protocol can make this problem hard to
reproduce.

mercurial/setdiscovery.py

@@ -165,7 +165,7 b' def findcommonheads(ui, local, remote,'
         ui.debug("all remote heads known locally\n")
         return (srvheadhashes, False, srvheadhashes,)
 
-    if sample and util.all(yesno):
+    if sample and len(ownheads) <= initialsamplesize and util.all(yesno):
         ui.note(_("all local heads known remotely\n"))
         ownheadhashes = dag.externalizeall(ownheads)
         return (ownheadhashes, True, srvheadhashes,)