revset: fix negative ancestor spec to not return changectx objects

The order was unstable because of this.

mercurial/revset.py

@@ -392,7 +392,7 b' def _childrenspec(repo, subset, x, n, or'
             if len(c) > 1:
                 raise error.RepoLookupError(
                     _("revision in set has more than one child"))
-            r = c[0]
+            r = c[0].rev()
         else:
             cs.add(r)
     return subset & cs

tests/test-revset.t

@@ -3131,9 +3131,11 b' parentrevspec'
   $ log 'merge()^^^'
   1
 
-  $ log '(merge() | 0)~-1'
+  $ hg debugrevspec -s '(merge() | 0)~-1'
+  * set:
+  <baseset+ [1, 7]>
+  1
   7
-  1
   $ log 'merge()~-1'
   7
   $ log 'tip~-1'