debugcommands: use a revset instead of dagutil

debugcommands: use a revset instead of dagutil

All this code was doing was finding the nodes that are heads
from the ancestors of an input set. This can easily be expressed
with a revset without having to go through dagutil.

This was the last use of ancestorset() outside of dagutil itself.

Differential Revision: https://phab.mercurial-scm.org/D4308

--- a/mercurial/debugcommands.py	Thu Aug 16 19:51:01 2018 +0000
+++ b/mercurial/debugcommands.py	Thu Aug 16 19:55:55 2018 +0000
@@ -42,7 +42,6 @@
     color,
     context,
     dagparser,
-    dagutil,
     encoding,
     error,
     exchange,
@@ -791,11 +790,10 @@
             if not opts.get('nonheads'):
                 ui.write(("unpruned common: %s\n") %
                          " ".join(sorted(short(n) for n in common)))
-                cl = repo.changelog
-                clnode = cl.node
-                dag = dagutil.revlogdag(cl)
-                all = dag.ancestorset(cl.rev(n) for n in common)
-                common = {clnode(r) for r in dag.headsetofconnecteds(all)}
+
+                clnode = repo.changelog.node
+                common = repo.revs('heads(::%ln)', common)
+                common = {clnode(r) for r in common}
         else:
             nodes = None
             if pushedrevs: