path: fix verbose relative() output

Fixes: https://github.com/nodejs/node/issues/5383
PR-URL: https://github.com/nodejs/node/pull/5389


Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Evan Lucas <evanlucas@me.com>