Fix messages displayed when diary is shown

* lisp/calendar/diary-lib.el (diary-list-entries): Display the
"...done" message only if there's no important message in the
echo-area.  (Bug#60795)

lisp/calendar/diary-lib.el

@@ -880,7 +880,10 @@ LIST-ONLY is non-nil, in which case it just returns the list."
                                    (original-date original-date))
                     (run-hooks 'diary-hook))))))
         (and temp-buff (buffer-name temp-buff) (kill-buffer temp-buff)))
-      (or d-incp (message "Preparing diary...done"))
+      (or d-incp
+          ;; Don't clobber messages displayed while preparing the diary.
+          (not (equal (current-message) "Preparing diary..."))
+          (message "Preparing diary...done"))
       diary-entries-list)))
 
 (defun diary-unhide-everything ()