changegroup: declare shallow flag in constructor Thus begins the process of better formalizing ellipses and shallow changegroup generation mode so it is tracked by cgpacker at construction time instead of bolted on after the fact by a wrapper function. Differential Revision: https://phab.mercurial-scm.org/D4086

changegroup: make some packer attributes private These methods and attributes are low level and should not be called or outside outside of instances. Indicate as such through naming. Differential Revision: https://phab.mercurial-scm.org/D4085