Backport commit 2ec41c415 from Emacs

* lisp/org-agenda.el (org-agenda-get-restriction-and-command):
Avoid add-to-list on local variables.

Avoid add-to-list on local variables
2ec41c415f39990561cc9da4c9bad0b69bfad489
Stefan Monnier
Wed Jan 4 00:40:45 2017 -0500

lisp/org-agenda.el

@@ -2941,7 +2941,7 @@ L   Timeline for current buffer         #   List stuck projects (!=configure)
 		  type (nth 2 entry)
 		  match (nth 3 entry))
 	    (if (> (length key) 1)
-		(add-to-list 'prefixes (string-to-char key))
+		(cl-pushnew (string-to-char key) prefixes :test #'equal)
 	      (setq line
 		    (format
 		     "%-4s%-14s"