changegroup: close progress in same function as it's started
changegroup.group() and changegroup.generatefiles() both currently
start progress (with topic "bundling"), but changegroup.generate()
closes the topic. Move the closing to the functions that start the
topic, so it's easier to see where the topic is started and closed.
This completes a move that seems to have been started in
0b564cf359a7
(bundle-ng: move progress handling out of the linkrev callback,
2013-05-10).