(/home/foo/bar/baz.pcb, /home/foo/heh.txt) = bar/baz.pcb
(/home/foo/heh.txt, /home/foo/bar/baz.pcb) = ../heh.txt
(/home/foo/bar/baz.pcb, /tmp/heh.txt) = ../home/foo/bar/baz.pcb
(/tmp/heh.txt, /home/foo/bar/baz.pcb) = ../../../tmp/heh.txt
