changegroup: remove 'getchangegroupraw' function There is no remaining caller for this function.

exchange: expand usage of getchangegroupraw The 'getchangegroupraw' is very simple (two lines) so we inline it in its only caller. This exposes the 'outgoing' object of the part generator function, allowing us to add information on the number of changesets contained in the part in a later changeset. Such information is useful for progress bar.