diff: make `hg diff --root=subrepo` work The root prefix is currently stripped before adding the subrepo prefix. It seems to me that if you run `hg diff --root=subrepo` and a subrepo/ path is visited (say "subrepo/foo"), then the path generated for the diff would initially be just "foo" and we then fail (if the developer warning is active) to strip the "subrepo/" from the path. It seems like we should first add the prefix in order to produce a path that's relative to the top-level repo, and then we can remove the root prefix from that. There are no tests for this, it seems, and I don't care enough to add one. Differential Revision: https://phab.mercurial-scm.org/D5895

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

test: use `printenv.py --line` in `test-static-http.t` This makes the output easier to update and read.