exchange: refactor control flow of _getbundlechangegrouppart()
The use of early return makes the control flow of this function much
easier to reason about IMO.
Differential Revision: https://phab.mercurial-scm.org/D4010
exchange: move _computeellipsis() from narrow
This is also referenced as part of the narrow changegroup code and
therefore needs to move to core before we can integrate the narrow
changegroup code into core.
Differential Revision: https://phab.mercurial-scm.org/D4009