patch: replace "prefix" and "relroot" arguments by "pathfn" (API) The two arguments serve a very similar purpose: "relroot" is stripped from the front of the path, and then "prefix" (a subrepo path) is added (also to the front). Passing in a function that does that is more generic and will make it easier to respect ui.relative-paths in later patches (don't worry, I'm not going to respect that option for regular patches, only for --stat). I'm deliberately not calling it "uipathfn", because it's generally for producing valid diffs (including when prefix is non-empty), so things like using backslash on Windows is not an option. Differential Revision: https://phab.mercurial-scm.org/D5894