revset: remove invalid value in the origin set

Same as the parents related revsets, origin had some invalid value in the
computed set. We remove them.

mercurial/revset.py

@@ -1167,6 +1167,7 def origin(repo, subset, x):
             src = prev
 
     o = set([_firstsrc(r) for r in args])
+    o -= set([None])
     return subset.filter(o.__contains__)
 
 def outgoing(repo, subset, x):