setdiscovery: use a revset for finding DAG heads in a subset

The march towards moving away from dagutil continues.

Like other patches moving us away from dagutil, there is the
potential for regressions to occur because revlogdag's
headsetofconnecteds() uses revlog.index, which doesn't take
filtering into account. The revset layer does. But no tests
fail, so we appear to be in the clear.

Differential Revision: https://phab.mercurial-scm.org/D4317

mercurial/setdiscovery.py

@@ -96,7 +96,7 b' def _updatesample(dag, revs, sample, qui'
                 dist.setdefault(p, d + 1)
                 visit.append(p)
 
-def _takequicksample(dag, revs, size):
+def _takequicksample(repo, dag, revs, size):
     """takes a quick sample of size <size>
 
     It is meant for initial sampling and focuses on querying heads and close
@@ -105,14 +105,16 b' def _takequicksample(dag, revs, size):'
     :dag: a dag object
     :revs: set of revs to discover
     :size: the maximum size of the sample"""
-    sample = dag.headsetofconnecteds(revs)
+    sample = set(repo.revs('heads(%ld)', revs))
+
     if len(sample) >= size:
         return _limitsample(sample, size)
     _updatesample(dag, None, sample, quicksamplesize=size)
     return sample
 
-def _takefullsample(dag, revs, size):
-    sample = dag.headsetofconnecteds(revs)
+def _takefullsample(repo, dag, revs, size):
+    sample = set(repo.revs('heads(%ld)', revs))
+
     # update from heads
     _updatesample(dag, revs, sample)
     # update from roots
@@ -242,7 +244,7 b' def findcommonheads(ui, local, remote,'
         if len(undecided) < targetsize:
             sample = list(undecided)
         else:
-            sample = samplefunc(dag, undecided, targetsize)
+            sample = samplefunc(local, dag, undecided, targetsize)
 
         roundtrips += 1
         progress.update(roundtrips)