scmutil: add a function to mark that files have been operated on Several places use scmutil.addremove as a means to declare that certain files have been operated on. This is ugly because: - addremove takes patterns relative to the cwd, not paths relative to the root, which means extra contortions for callers. - addremove doesn't make clear what happens to files whose status hasn't changed. This new method accepts filenames relative to the repo root, and has a much clearer contract. It also allows future modifications that do more with files whose status hasn't changed.