org-element: Fix interpreting links with % in description

* lisp/org-element.el (org-element-link-interpreter): % in description
  are confused with format string placeholders.  Escape them so as to
  fix the error.

* testing/lisp/test-org-element.el (test-org-element/link-interpreter):
  Add test.

Reported-by: Daniel Clemente <n142857@gmail.com>
<http://permalink.gmane.org/gmane.emacs.orgmode/109878>

lisp/org-element.el

@@ -3194,7 +3194,12 @@ CONTENTS is the contents of the object, or nil."
 		   ;; cases.  This is also the default syntax when the
 		   ;; property is not defined, e.g., when the object
 		   ;; was crafted by the user.
-		   ((guard contents) (format "[[%%s][%s]]" contents))
+		   ((guard contents)
+		    (format "[[%%s][%s]]"
+			    ;; Since this is going to be used as
+			    ;; a format string, escape percent signs
+			    ;; in description.
+			    (replace-regexp-in-string "%" "%%" contents)))
 		   ((or `bracket
 			`nil
 			(guard (member type '("coderef" "custom-id" "fuzzy"))))

testing/lisp/test-org-element.el

@@ -3026,7 +3026,10 @@ DEADLINE: <2012-03-29 thu.> SCHEDULED: <2012-03-29 thu.> CLOSED: [2012-03-29 thu
 		 "http://orgmode.org\n"))
   ;; Angular links.
   (should (equal (org-test-parse-and-interpret "<http://orgmode.org>")
-		 "<http://orgmode.org>\n")))
+		 "<http://orgmode.org>\n"))
+  ;; Pathological case: link with a %-sign in description.
+  (should (equal (org-test-parse-and-interpret "[[file://path][%s]]")
+		 "[[file://path][%s]]\n")))
 
 (ert-deftest test-org-element/macro-interpreter ()
   "Test macro interpreter."