revset: for x^2, do not take null as a valid p2 revision

Since we don't count null p2 revision as a parent, x^2 should never return
null even if null is explicitly populated.

mercurial/revset.py

@@ -1621,7 +1621,7 b' def parentspec(repo, subset, x, n, order'
             ps.add(cl.parentrevs(r)[0])
         elif n == 2:
             parents = cl.parentrevs(r)
-            if len(parents) > 1:
+            if parents[1] != node.nullrev:
                 ps.add(parents[1])
     return subset & ps
 

tests/test-revset.t

@@ -2750,6 +2750,7 b' parentrevspec'
   5
   $ log 'merge()^2'
   4
+  $ log '(not merge())^2'
   $ log 'merge()^^'
   3
   $ log 'merge()^1^'