revset: added __nonzero__ method to lazyset

Now it doesn't have to go through all the set and can return lazily as soon as
it finds one element.

mercurial/revset.py

@@ -2155,6 +2155,11 b' class lazyset(object):'
         l = baseset([r for r in self])
         return l + baseset(x)
 
+    def __nonzero__(self):
+        for r in self:
+            return True
+        return False
+
     def __len__(self):
         # Basic implementation to be changed in future patches.
         l = baseset([r for r in self])