scmutil: introduce a new backuppath() to replace origpath() Unlike most functions in our codebase, origpath() takes a path that is relative to cwd. This commit introduces a replacement for origpath(). The new function takes a path that is relative to the repo root. There is a lot of duplication between the two, but I intend to remove origpath() within the next few commits, so it won't be a maintenance burden. origpath() is also a little weird in that it returns either a a cwd-relative path or an absolute path. It needs to be able to return a path outside the repo, so it makes sense that it can return an absolute path. However, it would be simpler to always return an absolute path. The new function does that. Differential Revision: https://phab.mercurial-scm.org/D5851