revset: add a default argument for baseset.__init__

We are now able to create empty baseset using `baseset()` as we are able to
create empty list with `list()`.

mercurial/revset.py

@@ -2174,7 +2174,7 b' class baseset(list):'
 
     Every method in this class should be implemented by any smartset class.
     """
-    def __init__(self, data):
+    def __init__(self, data=()):
         super(baseset, self).__init__(data)
         self._set = None