bundle2: add a ``newpart`` method to ``bundle20``

Creating new parts is the most common operation people do when exposed to a
bundler. We create a dedicated method on the bundler object for it. This will
simplify the code and also avoid having to import the ``mercurial.bundle2``
module in multiple places.

One part creators have been updated for testing purpose.

mercurial/bundle2.py

@@ -408,6 +408,11 b' class bundle20(object):'
         part.id = len(self._parts) # very cheap counter
         self._parts.append(part)
 
+    def newpart(self, typeid, *args, **kwargs):
+        """create a new part for the containers"""
+        part = bundlepart(typeid, *args, **kwargs)
+        return part
+
     # methods used to generate the bundle2 stream
     def getchunks(self):
         self.ui.debug('start emission of %s stream\n' % _magicstring)
@@ -707,7 +712,7 b' def handlechangegroup(op, inpart):'
     if op.reply is not None:
         # This is definitly not the final form of this
         # return. But one need to start somewhere.
-        part = bundlepart('b2x:reply:changegroup', (),
+        part = op.reply.newpart('b2x:reply:changegroup', (),
                            [('in-reply-to', str(inpart.id)),
                             ('return', '%i' % ret)])
         op.reply.addpart(part)