changegroup: replace changegroupsubset with makechangegroup
As part of getting rid of all the permutations of changegroup creation, let's
remove changegroupsubset and call makechangegroup instead. This moves the
responsibility of creating the outgoing set to the caller, but that seems like a
relatively reasonable unit of functionality for the caller to have to care about
(i.e. what commits should be bundled).
Differential Revision: https://phab.mercurial-scm.org/D665
Greg Ward, author of the original bfiles extension
Na'Tosha Bard of Unity Technologies
Fog Creek Software
Special thanks to the University of Toronto and the UCOSP program