generatorset: get list-based fast iterations after the generator is consumed

When all revisions are known, we shortcut most of the class logic to use list
iteration instead. The cost of the sort is expected to be non-significant. The
list creation and sorting could be done lazily in the future. We have to copy
the list to not break existing iterator created before we finished consuming the
generator.

mercurial/revset.py

@@ -2621,6 +2621,7 b' class generatorset(abstractsmartset):'
         gen: a generator producing the values for the generatorset.
         """
         self._gen = gen
+        self._asclist = None
         self._cache = {}
         self._genlist = []
         self._finished = False
@@ -2712,7 +2713,13 b' class generatorset(abstractsmartset):'
             cache[item] = True
             genlist(item)
             yield item
-        self._finished = True
+        if not self._finished:
+            self._finished = True
+            asc = self._genlist[:]
+            asc.sort()
+            self._asclist = asc
+            self.fastasc = asc.__iter__
+            self.fastdesc = asc.__reversed__
 
     def set(self):
         return self