org.el: Filter agenda by persistent tags

* org.el (org-agenda-prepare-buffers): Add tags defined in
org-tag-persistent-alist to org-tag-alist-for-agenda.

This makes tag hotkeys defined in `org-tag-persistent-alist' appear when
the user invokes `org-agenda-filter-by-tag-refine'.

TINYCHANGE

lisp/org.el

@@ -17985,7 +17985,10 @@ When a buffer is unmodified, it is just killed.  When modified, it is saved
 	    (setq org-drawers-for-agenda
 		  (append org-drawers-for-agenda org-drawers))
 	    (setq org-tag-alist-for-agenda
-		  (org-uniquify (append org-tag-alist-for-agenda org-tag-alist)))
+		  (org-uniquify 
+		   (append org-tag-alist-for-agenda 
+			   org-tag-alist
+			   org-tag-persistent-alist)))
 	    (if org-group-tags
 		(setq org-tag-groups-alist-for-agenda
 		      (org-uniquify-alist