obsstore: add a __len__ method

We can already use "for mark in store:" it make sense to allow
"len(store)" too.

mercurial/obsolete.py

@@ -247,6 +247,9 b' class obsstore(object):'
     def __iter__(self):
         return iter(self._all)
 
+    def __len__(self):
+        return len(self._all)
+
     def __nonzero__(self):
         return bool(self._all)