bundle2: extract a _payloadchunks method for part

We are preparing streaming capability for part. So the generation of payload
chunk will becomes more complex. We extract this part in its own function before
any changes.

mercurial/bundle2.py

@@ -549,13 +549,20 b' class part(object):'
         yield _pack(_fpartheadersize, len(headerchunk))
         yield headerchunk
         ## payload
+        for chunk in self._payloadchunks():
+            yield _pack(_fpayloadsize, len(chunk))
+            yield chunk
+        # end of payload
+        yield _pack(_fpayloadsize, 0)
+
+    def _payloadchunks(self):
+        """yield chunks of a the part payload
+
+        Exists to handle the different methods to provide data to a part."""
         # we only support fixed size data now.
         # This will be improved in the future.
         if len(self.data):
-            yield _pack(_fpayloadsize, len(self.data))
             yield self.data
-        # end of payload
-        yield _pack(_fpayloadsize, 0)
 
 @parthandler('changegroup')
 def handlechangegroup(op, inpart):