revset: avoid in loop lookup in _generatorset._consumegen

Python lookups are slow, so do all lookup outside of the for loop.

This provide a small but still significant speedup:

revset  #0: 0::
0) wall 0.063258 comb 0.060000 user 0.060000 sys 0.000000 (best of 100)
1) wall 0.057776 comb 0.050000 user 0.050000 sys 0.000000 (best of 100)

mercurial/revset.py

@@ -2672,9 +2672,11 b' class _generatorset(object):'
             i += 1
 
     def _consumegen(self):
+        cache = self._cache
+        genlist = self._genlist.append
         for item in self._gen:
-            self._cache[item] = True
-            self._genlist.append(item)
+            cache[item] = True
+            genlist(item)
             yield item
         self._finished = True