Fix bug that erases org buffer when calling agenda via org-agenda-open-link.

[My apologies, but I'm afraid my first attempt at this patch mistook a
necessary second check for redundancy. Here is an improved version.]

* lisp/org-agenda.el (org-prepare-agenda): If the agenda is called
  from within the agenda via an elisp link, such as
  [[elisp:(org-agenda-list)]], org-prepare-agenda erases the buffer of
  the file containing the link, since that buffer is current during
  org-prepare agenda (due to a with-current-buffer in
  org-agenda-open-link). An additional test now ensures that the
  agenda buffer is in fact current when the buffer is erased and local
  variables for the agenda are set.

lisp/org-agenda.el

@@ -2816,7 +2816,11 @@ the global options and expect it to be applied to the entire view.")
 	(switch-to-buffer-other-frame abuf))
        ((equal org-agenda-window-setup 'reorganize-frame)
 	(delete-other-windows)
-	(org-switch-to-buffer-other-window abuf))))
+	(org-switch-to-buffer-other-window abuf)))
+      ;; additional test in case agenda is invoked from within agenda
+      ;; buffer via elisp link
+      (unless (equal (current-buffer) abuf)
+	(switch-to-buffer abuf)))
     (setq buffer-read-only nil)
     (let ((inhibit-read-only t)) (erase-buffer))
     (org-agenda-mode)