org.el: Include `org-tag-alist' in the list for tag completions.

* org.el (org-set-tags): Include `org-tag-alist' in the list
of possible completions, even when there are tags defined in
the buffer.

Thanks to Mike McLean for reporting this.

lisp/org.el

@@ -13389,7 +13389,8 @@ With prefix ARG, realign all tags in headings in the current buffer."
 	;; Get a new set of tags from the user
 	(save-excursion
 	  (setq table (append org-tag-persistent-alist
-			      (or org-tag-alist (org-get-buffer-tags))
+			      org-tag-alist
+			      (org-get-buffer-tags)
 			      (and
 			       org-complete-tags-always-offer-all-agenda-tags
 			       (org-global-tags-completion-table