generatorset: make it possible to use gen as fastasc or fastdesc

We gain a parameter to inform that the generator is ascending or descending. If
the generator is ordered, it is also used for the `fastasc` or `fastdesc`
version.

The _ascgeneratorset and _descgeneratorset class will be removed soon.

mercurial/revset.py

@@ -2622,7 +2622,7 b' class _generatorset(abstractsmartset):'
     This class does not duck-type baseset and it's only supposed to be used
     internally
     """
-    def __init__(self, gen):
+    def __init__(self, gen, iterasc=None):
         """
         gen: a generator producing the values for the generatorset.
         """
@@ -2630,6 +2630,11 b' class _generatorset(abstractsmartset):'
         self._cache = {}
         self._genlist = baseset([])
         self._finished = False
+        if iterasc is not None:
+            if iterasc:
+                self.fastasc = self.__iter__
+            else:
+                self.fastdesc = self.__iter__
 
     def __nonzero__(self):
         for r in self: