revset: leverage orset() to flatten ancestor() arguments

This also makes orset() accept an empty argument because nullary ancestor()
call is valid. That's not the case for orset(), but should be okay.

mercurial/revset.py

@@ -203,6 +203,8 b' def _orsetlist(repo, subset, xs, order):'
 
 def orset(repo, subset, x, order):
     xs = getlist(x)
+    if not xs:
+        return baseset()
     if order == followorder:
         # slow path to take the subset order
         return subset & _orsetlist(repo, fullreposet(repo), xs, anyorder)
@@ -309,17 +311,12 b' def ancestor(repo, subset, x):'
     Will return empty list when passed no args.
     Greatest common ancestor of a single changeset is that changeset.
     """
-    l = getlist(x)
-    rl = fullreposet(repo)
     anc = None
-
-    # (getset(repo, rl, i) for i in l) generates a list of lists
-    for revs in (getset(repo, rl, i) for i in l):
-        for r in revs:
-            if anc is None:
-                anc = repo[r]
-            else:
-                anc = anc.ancestor(repo[r])
+    for r in orset(repo, fullreposet(repo), x, order=anyorder):
+        if anc is None:
+            anc = repo[r]
+        else:
+            anc = anc.ancestor(repo[r])
 
     if anc is not None and anc.rev() in subset:
         return baseset([anc.rev()])