Fix bug with numjobs > 1, directory and filename given

Reported by gurudas pai <gurudas.pai@oracle.com>

If numjobs > 1, add_job() will append the directory name
several times. Fix this by doing the directory append in
add_file() instead, it cleans up the code as well.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>