revset: fix a crash in parents() when 'wdir()' is in the set

The crash was "TypeError: expected string or Unicode object, NoneType found"
down in revlog.parentrevs().  This fixes heads() too (which is where I found
it.)

mercurial/revset.py

@@ -1468,6 +1468,9 b' def parents(repo, subset, x):'
         ps = set()
         cl = repo.changelog
         for r in getset(repo, fullreposet(repo), x):
+            if r is None:
+                ps.update(p.rev() for p in repo[r].parents())
+            else:
             ps.update(cl.parentrevs(r))
     ps -= set([node.nullrev])
     return subset & ps

tests/test-add.t

@@ -75,6 +75,13 b' should fail'
 
   $ hg ci -m 0 --traceback
 
+  $ hg log -r "heads(. or wdir() & file('**'))"
+  changeset:   0:* (glob)
+  tag:         tip
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     0
+  
 should fail
 
   $ hg add a
@@ -99,6 +106,15 b' should fail'
   M a
   ? a.orig
 
+wdir doesn't cause a crash, and can be dynamically selected if dirty
+
+  $ hg log -r "heads(. or wdir() & file('**'))"
+  changeset:   2:*+ (glob)
+  parent:      2:* (glob)
+  parent:      1:* (glob)
+  user:        test
+  date:        * (glob)
+  
 should fail
 
   $ hg add a