try to fix test-inherit-mode on HFS+ HFS+ doesn't like setgid directories, so avoid them in most of the test and do a quick check that doesn't change the output when everything's OK.

cmdutil.commit: use relative paths in the error messages This is more consistent with other error messages and requires less makeup in test-commit and test-symlink-basic.

util.pathto: return '.' instead of an empty string This could happen with something like pathto(repo.root, 'foo/bar', 'foo/bar')