patch: let caller pass in root-filtering matcher (API) The --root option to `hg diff` does two things: * Shows paths relative to the given root * Filters paths by the given root, including copy sources The root argument is passed through down to patch.diff(). I feel like we can make patch.diff() more generic by not passing down the root argument, but instead pass: * A function for taking a repo-relative path and printing it. I want to reuse this for showing cwd-relative paths later. This is the actual motivation for this patch. * A matcher that's already been filtered by the root argument * A second matcher that filters the copy sources This is one step towards that. Differential Revision: https://phab.mercurial-scm.org/D5892