org-find-exact-heading-in-directory: Use full path

* lisp/org.el (org-find-exact-heading-in-directory): Use full path for
files.

When the path is not included, `org-find-exact-heading-in-directory'
will fail when the current value of `default-directory' is different
from the argument DIR.

TINYCHANGE
org.texi: Fix whitespaces

TINYCHANGE