py3: use next() to obtain next item from inner generator of generatorset

py3: use next() to obtain next item from inner generator of generatorset

.next attribute does not exist on Python 3. As this function seems to really
care about the overhead of the Python interpreter, I follow the way of micro
optimization.

--- a/mercurial/smartset.py	Mon Mar 13 08:44:57 2017 -0700
+++ b/mercurial/smartset.py	Mon Mar 13 08:53:31 2017 -0700
@@ -818,15 +818,15 @@
         # Getting rid of it would provide an about 15% speed up on this
         # iteration.
         genlist = self._genlist
-        nextrev = self._consumegen().next
-        _len = len # cache global lookup
+        nextgen = self._consumegen()
+        _len, _next = len, next # cache global lookup
         def gen():
             i = 0
             while True:
                 if i < _len(genlist):
                     yield genlist[i]
                 else:
-                    yield nextrev()
+                    yield _next(nextgen)
                 i += 1
         return gen()