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.