org-babel-result-to-file: Fix results file inside attachment dir

* lisp/ob-core.el (org-babel-result-to-file): When file path in the
result contains attachment dir, reduce the path to relative file name
when generating attachment: link.
* testing/lisp/test-ob.el (test-ob-core/dir-attach): Add test.

Reported-by: Johan Tolö <johan@toloe.se>
Link: https://orgmode.org/list/875yesnia6.fsf@toloe.se

lisp/ob-core.el

@@ -2692,8 +2692,8 @@ specified as an an \"attachment:\" style link."
            (same-directory?
 	    (and base-file-name
 	         (not (string= (expand-file-name default-directory)
-			       (expand-file-name
-			        base-directory)))))
+			     (expand-file-name
+			      base-directory)))))
            (request-attachment (eq type 'attachment))
            (attach-dir (let* ((default-directory base-directory)
                               (dir (org-attach-dir nil t)))
@@ -2709,7 +2709,9 @@ specified as an an \"attachment:\" style link."
                 ((and 'attachment (guard in-attach-dir)) "attachment")
                 (_ "file"))
               (if (and request-attachment in-attach-dir)
-                  (file-relative-name result-file-name)
+                  (file-relative-name
+                   result-file-name
+                   (file-name-as-directory attach-dir))
 	        (if (and default-directory
 		         base-file-name same-directory?)
 		    (if (eq org-link-file-path-type 'adaptive)

testing/lisp/test-ob.el

@@ -1954,6 +1954,23 @@ nil
      (file-exists-p (format "%s/test.txt" (org-attach-dir nil t)))
      (string= (buffer-substring-no-properties (point) (line-end-position))
               "[[attachment:test.txt]]"))))
+  ;; Strip attach dir from the file path.
+  (should
+   (org-test-with-temp-text-in-file
+    "* heading
+:PROPERTIES:
+:DIR:      custom-attach-dir
+:END:
+
+<point>#+begin_src elisp :results value file
+\"custom-attach-dir/test.txt\"
+#+end_src"
+    (message "DIR: %s" (org-attach-dir t))
+    (org-babel-execute-src-block)
+    (goto-char (org-babel-where-is-src-block-result))
+    (forward-line)
+     (string= (buffer-substring-no-properties (point) (line-end-position))
+              "[[attachment:test.txt]]")))
   (should-error
    (org-test-with-temp-text-in-file
     "* 'attach with no ID or DIR