changegroup: replace getsubset with makechangegroup The current changegroup APIs are a bit of a mess. Currently you can use getsubsetraw, getsubset, changegroupsubset, getlocalchangegroupraw, getchangegroup, and getlocalchangroup to produce changegroups. This patch is the beginning of a refactor to boil all of that away to just makechangegroup and makestream. The first step adds the new functions and replaces getsubset function with them. Differential Revision: https://phab.mercurial-scm.org/D664