changegroup: move deltaparent() from narrow
I'm not keen on performing the attribute sniff to test for
presence of ellipses mode: I'd rather we use a separate packer
instance that was ellipses mode specific. But I've tried to
formalize a better API without narrow in core and I can't
make sense of all the monkeypatching. My goal is to inline
as much of the monkeypatching as possible then refactor the
changegroup generation API.
We add this code to the cg2packer because narrow doesn't work
with cg1.
Differential Revision: https://phab.mercurial-scm.org/D4062