changegroup: move changelogdone into cgpacker
Looking at what it is used for, it feels like there is a better
way to implement all this. So recording a TODO to track that.
Differential Revision: https://phab.mercurial-scm.org/D4087
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