setdiscovery: don't use dagutil to compute heads

This is a relatively trivial operation to perform. We don't
need to use dagutil.

This brings us one step closer to eliminating dagutil.

We still need to populate the heads on revlogdag because later
functionality relies on it.

Differential Revision: https://phab.mercurial-scm.org/D4310

mercurial/setdiscovery.py

@@ -144,16 +144,17 b' def findcommonheads(ui, local, remote,'
     cl = local.changelog
     clnode = cl.node
     clrev = cl.rev
-    localsubset = None
 
     if ancestorsof is not None:
-        localsubset = [clrev(n) for n in ancestorsof]
-    dag = dagutil.revlogdag(cl, localsubset=localsubset)
+        ownheads = [clrev(n) for n in ancestorsof]
+    else:
+        ownheads = [rev for rev in cl.headrevs() if rev != nullrev]
+
+    dag = dagutil.revlogdag(cl, localsubset=ownheads)
 
     # early exit if we know all the specified remote heads already
     ui.debug("query 1; heads\n")
     roundtrips += 1
-    ownheads = dag.heads()
     sample = _limitsample(ownheads, initialsamplesize)
     # indices between sample and externalized version must match
     sample = list(sample)