revset: remove nullrev from the bookmark computation

Same as for other revset we sanitize the content of the set to be able to rely
on it more.

mercurial/revset.py

@@ -467,6 +467,7 b' def bookmark(repo, subset, x):'
     else:
         bms = set([repo[r].rev()
                    for r in repo._bookmarks.values()])
+    bms -= set([node.nullrev])
     return subset.filter(bms.__contains__)
 
 def branch(repo, subset, x):