revset: make commonancestors(x + x^) be ::(x^), not ::x

Differential Revision: https://phab.mercurial-scm.org/D4738

mercurial/revset.py

@@ -631,13 +631,11 b' def commonancestors(repo, subset, x):'
     in the repo (whereas we want "::x *and* ::y").
 
     """
-    # only wants the heads of the set passed in
-    h = heads(repo, fullreposet(repo), x, anyorder)
-    if not h:
+    startrevs = getset(repo, fullreposet(repo), x, order=anyorder)
+    if not startrevs:
         return baseset()
-    for r in h:
+    for r in startrevs:
         subset &= dagop.revancestors(repo, baseset([r]))
-
     return subset
 
 @predicate('contains(pattern)', weight=100)

tests/test-revset.t

@@ -1069,7 +1069,6 b' test common ancestors'
   2
   4
   8
-  9
 
 test ancestor variants of empty revision