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