setdiscovery: use iterbatch interface instead of batch

It's a little more concise, and gives us some simple test coverage.

mercurial/setdiscovery.py

@@ -147,12 +147,11 b' def findcommonheads(ui, local, remote,'
     sample = _limitsample(ownheads, initialsamplesize)
     # indices between sample and externalized version must match
     sample = list(sample)
-    batch = remote.batch()
-    srvheadhashesref = batch.heads()
-    yesnoref = batch.known(dag.externalizeall(sample))
+    batch = remote.iterbatch()
+    batch.heads()
+    batch.known(dag.externalizeall(sample))
     batch.submit()
-    srvheadhashes = srvheadhashesref.value
-    yesno = yesnoref.value
+    srvheadhashes, yesno = batch.results()
 
     if cl.tip() == nullid:
         if srvheadhashes != [nullid]: