revset: speed up existence checks for ordered filtered sets

Previously, calling 'if foo:' on a ordered filtered set would start iterating in
whatever the current direction was and return if a value was available. If the
current direction was ascending, but the set had a fastdesc available, this
meant we did a lot more work than necessary.

If this was applied without my previous max/min fixes, it would improve max()
performance (this was my first attempt at fixing the issue). Since those
previous fixes went in though, this doesn't have a visible benefit in the
benchmarks, but it does seem clearly better than it was before so I think it
should still go in.

mercurial/revset.py

@@ -3225,7 +3225,12 class filteredset(abstractsmartset):
         return lambda: self._iterfilter(it())
 
     def __nonzero__(self):
-        for r in self:
+        it = self
+        fast = self.fastasc or self.fastdesc
+        if fast:
+            it = fast()
+
+        for r in it:
             return True
         return False