path: fix win32 relative() when "to" is a prefix

when the basename of "to" was a prefix of the basename of "from" win32
relative() would miss including it in the result

Fixes: https://github.com/nodejs/node/issues/5447
PR-URL: https://github.com/nodejs/node/pull/5456


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Roman Reiss <me@silverwind.io>