ox: Fix regression in INCLUDE keywords
* lisp/ox.el (org-export--prepare-file-contents): Activate Org mode in
temporary buffer so all regexps are set. Also, be more strict when
updating links, i.e., do not bother if both includer and includee
belong to the same directory, or if there's no includer at all.
Eventually, only update links within lines specifications, if any.
* testing/lisp/test-ox.el (test-org/expand-include/links): Add tests.
Reported-by: Kaushal Modi <kaushal.modi@gmail.com>
<http://lists.gnu.org/r/emacs-orgmode/2018-03/msg00394.html>