setdiscovery: use revsets for computing a subset's heads and roots

revlogdag.headsetofconnecteds() obtains the set of DAG heads in a
given set of revs.

revlogdag.inverse() inverts the DAG order and makes
headsetofconnecteds() obtain the DAG roots in a given subset.

Both of these can be expressed with a revset.

Like other patches in this series, revlogdag uses revlog.index
and thus doesn't take filtering into account. Revsets do. So there
is a chance for regressions with this change. But no tests fail.
And I think this code should take filtering into account since
hidden changesets shouldn't factor into discovery (unless operating
on the hidden repository).

Differential Revision: https://phab.mercurial-scm.org/D4321

mercurial/setdiscovery.py

@@ -114,11 +114,11 b' def _takefullsample(repo, dag, headrevs,'
     sample = set(repo.revs('heads(%ld)', revs))
 
     # update from heads
-    _updatesample(dag, revs, dag.headsetofconnecteds(revs), sample)
+    revsheads = set(repo.revs('heads(%ld)', revs))
+    _updatesample(dag, revs, revsheads, sample)
     # update from roots
-    inverteddag = dag.inverse()
-    _updatesample(inverteddag, revs, inverteddag.headsetofconnecteds(revs),
-                  sample)
+    revsroots = set(repo.revs('roots(%ld)', revs))
+    _updatesample(dag.inverse(), revs, revsroots, sample)
     assert sample
     sample = _limitsample(sample, size)
     if len(sample) < size: