changegroup: replace getchangegroup with makechangegroup
As part of reducing the number of changegroup creation APIs, let's replace
getchangegroup with calls to makechangegroup. This is mostly a drop in
replacement, but it does change the version specifier to be required, so it's
more obvious which callers are creating old version 1 changegroups still.
Differential Revision: https://phab.mercurial-scm.org/D669