I've fixed the implementation of 
`org-export--prepare-file-contents` so
that links are relative to the included file. A patch is attached.

I've also created two org files in the test examples directory:
"includer-with-links.org" and "subdir/includee-with-links.org". My 
goal
was to add a test in "test-ox.el" `test-org-export/expand-include` 
that
checks whether, after exporting the "includer-with-links.org" file 
to
Org, the headings *Source and *Target are the same.

I did not figure out how to do this cleanly, so any help 
implementing
the test itself would be greatly appreciated.

Regards,

Daniel