fixed git.Tree.__len__ arguments

(cherry picked from commit 657444d75d7b2d6913c2d491841b9bd70c8bd289)

Author: k0001

lib/git/tree.py

@@ -89,7 +89,7 @@ def __getitem__(self, key):
     def __iter__(self):
         return iter(self._contents)
 
-    def __len__(self, keys):
+    def __len__(self):
         return len(self._contents)
 
     def __contains__(self, key):