Silence a warning caused by $dir1->subsumes($dir2) when $dir2
subsumes $dir1. [Chia-liang Kao]Work around a File::Spec->abs2rel bug when its two arguments are
the same. It returns an empty string rather than curdir(), so in
Path::Class, dir()->absolute->relative returned '/' rather than '.'
(or their equivalent on non-unix platforms. [Spotted by David Golden]