revset: make generatorset.__nonzero__ lazy

The 'for r in self:' call could trigger full consumption of the generator while
we only need a single value. We also fast path if a single value got already
computed. See inline comment for more details.

This provide massive speedup for lazy operation using boolean testing.

max(::tip)
e5b507efb36e) wall 0.055609 comb 0.060000 user 0.060000 sys 0.000000 (best of 100)
after change) wall 0.000109 comb 0.000000 user 0.000000 sys 0.000000 (best of 19146)

mercurial/revset.py

@@ -3137,7 +3137,12 b' class generatorset(abstractsmartset):'
                 self.__contains__ = self._desccontains
 
     def __nonzero__(self):
-        for r in self:
+        # Do not use 'for r in self' because it will enforce the iteration
+        # order (default ascending), possibly unrolling a whole descending
+        # iterator.
+        if self._genlist:
+            return True
+        for r in self._consumegen():
             return True
         return False