I already answered this question up above. The code that uses this result is:
https://github.com/mojombo/jekyll/blob/master/lib/jekyll/generators/pagination.rb#L40

If you wanted this function to return the whole path then you need to rework `paginate` to not join this with the `page.dir`

This is also already currently tested with your tests:
https://github.com/mojombo/jekyll/blob/master/features/pagination.feature#L33