Fix completion when selecting more than one tag

* lisp/org.el (org-fast-tag-selection): Make sure to set local variable
  buffer-tags to actual buffer-tags before pushing selected tag to the
  list; otherwise, the subsequent completion list will be limited to the
  one selected tag.

lisp/org.el

@@ -15315,7 +15315,8 @@ Returns the new tags string, or nil to not change the current settings."
 				"Tag: "
 				(or buffer-tags
 				    (with-current-buffer buf
-				      (org-get-buffer-tags)))))
+				      (setq buffer-tags
+					    (org-get-buffer-tags))))))
 		    (quit (setq tg "")))
 		  (when (string-match "\\S-" tg)
 		    (add-to-list 'buffer-tags (list tg))