baseset: keep the input set around

Baseset needs a list to operate, but will convert that list back to a set for
membership testing. It seems a bit silly to convert the set into a list to
convert it back afterward.

mercurial/revset.py

@@ -2967,6 +2967,8 b' class baseset(abstractsmartset):'
     """
     def __init__(self, data=()):
         if not isinstance(data, list):
+            if isinstance(data, set):
+                self._set = data
             data = list(data)
         self._list = data
         self._ascending = None