bundle2: remove an explicit packing

The final writing of the empty part was done explicitly. We now using proper
pack call using symbolic constant. This open simple change in the bundle2
format.

mercurial/bundle2.py

@@ -447,7 +447,7 b' class bundle20(object):'
             for chunk in part.getchunks():
                 yield chunk
         self.ui.debug('end of bundle\n')
-        yield '\0\0'
+        yield _pack(_fpartheadersize, 0)
 
     def _paramchunk(self):
         """return a encoded version of all stream parameters"""