match: make sure `root` argument is always an absolute path (API) The `root` argument should already be an absolute path, but we had tests that passed a relative path. This patch fixes up the tests and adds an assertion. This assumes that `os.path.isabs('/repo')` will be `True` on all platforms we care to run tests on. Augie tested for me that it does work on Windows, so that's good enough for me. Differential Revision: https://phab.mercurial-scm.org/D7649