manifest: add __nonzero__ method

This adds a __nonzero__ method to manifestdict. This isn't strictly necessary in
the vanilla Mercurial implementation, since Python will handle nonzero checks by
using __len__, but having it implemented here makes it easier for alternative
implementations to implement __nonzero__ and have them be plug-n-play with the
normal implementation.

mercurial/manifest.py

@@ -422,6 +422,11 b' class manifestdict(object):'
     def __len__(self):
         return len(self._lm)
 
+    def __nonzero__(self):
+        # nonzero is covered by the __len__ function, but implementing it here
+        # makes it easier for extensions to override.
+        return len(self._lm) != 0
+
     def __setitem__(self, key, node):
         self._lm[key] = node, self.flags(key, '')