revset: remove nullrev from set computed in parents()

The old code relied on the subset contents to get rid of invalid values. We would
like to be able to rely more on the computation in parents() so we filter out
the invalid value.

mercurial/revset.py

@@ -1237,6 +1237,7 b' def parents(repo, subset, x):'
         cl = repo.changelog
         for r in getset(repo, spanset(repo), x):
             ps.update(cl.parentrevs(r))
+    ps -= set([node.nullrev])
     return baseset(ps) & subset
 
 def parentspec(repo, subset, x, n):