revset: add a `__nonzero__` to baseset

We are about to add a base class for `baseset` with an abstract `__nonzero__`
method. So we need this method to be explicitly defined to avoid issues. The
built-in list object apparently does not have a `__nonzero__` and relies on
`__len__` for this purpose?

mercurial/revset.py

@@ -2238,6 +2238,9 b' class baseset(list):'
     def __contains__(self):
         return self.set().__contains__
 
+    def __nonzero__(self):
+        return bool(len(self))
+
     def __sub__(self, other):
         """Returns a new object with the substraction of the two collections.