Add __setitem__ to DictRefsContainer.

BaseRefsContainer assumes __setitem__, but it is not defined in the base
class. This was not a problem previously because DictRefsContainer was
not widely used, but that will change.

Change-Id: Ica0786e63ed2ef301c63671a5f3637048c0338f9

dulwich/repo.py

@@ -270,6 +270,9 @@ class DictRefsContainer(RefsContainer):
     def read_loose_ref(self, name):
         return self._refs[name]
 
+    def __setitem__(self, name, value):
+        self._refs[name] = value
+
 
 class DiskRefsContainer(RefsContainer):
     """Refs container that reads refs from disk."""