exchange: move simple narrow changegroup generation from extension The narrow extension completely replaced the function generating the changegroup part when a narrow changegroup was requested. Previous commits have taught the in-core changegroup code how to filter files based on a matcher. This commit teaches the in-core bundle2 part generation code to construct a matcher based on arguments. It will also emit a bundle2 part describing the narrow spec. I believe the changegroup part generation code in the narrow extension is now limited to ellipsis serving mode. i.e. core is now capable of narrow changegroup generation when ellipsis mode is disabled. Differential Revision: https://phab.mercurial-scm.org/D4014