org.el: Complete tags from both global and buffer local

* lisp/org.el (org-fast-tag-selection): Merge buffer local tags with
global alist of tags. And it obeys the option
org-complete-tags-always-offer-all-agenda-tags.

* doc/org-manual.org: Update the TAB key doc in tags selection UI.

* etc/ORG-NEWS: Mention the change in org-set-tags-command.

doc/org-manual.org

@@ -4860,9 +4860,10 @@ In this interface, you can also use the following special keys:
 
   #+kindex: TAB
   Enter a tag in the minibuffer, even if the tag is not in the
-  predefined list.  You can complete on all tags present in the
-  buffer.  You can also add several tags: just separate them with
-  a comma.
+  predefined list.  You can complete on all tags present in the buffer
+  and globally pre-defined tags from ~org-tag-alist~ and
+  ~org-tag-persistent-alist~.  You can also add several tags: just
+  separate them with a comma.
 
 - {{{kbd(SPC)}}} ::
 

etc/ORG-NEWS

@@ -149,6 +149,13 @@ Example:
 A new =u= mode flag for Calc formulas in Org tables has been added to
 enable Calc units simplification mode.
 
+*** =org-set-tags-command= select tags from ~org-global-tags-completion-table~
+
+Let ~org-set-tags-command~ TAB fast tag completion interface complete
+tags including from both buffer local and user defined persistent
+global list (~org-tag-alist~ and ~org-tag-persistent-alist~).  Now
+option ~org-complete-tags-always-offer-all-agenda-tags~ is honored.
+
 ** Miscellaneous
 *** =org-goto-first-child= now works before first heading
 

lisp/org.el

@@ -12139,7 +12139,7 @@ Returns the new tags string, or nil to not change the current settings."
 				  fulltable))))
 	 (buf (current-buffer))
 	 (expert (eq org-fast-tag-selection-single-key 'expert))
-	 (buffer-tags nil)
+	 (tab-tags nil)
 	 (fwidth (+ maxlen 3 1 3))
 	 (ncol (/ (- (window-width) 4) fwidth))
 	 (i-face 'org-done)
@@ -12274,16 +12274,21 @@ Returns the new tags string, or nil to not change the current settings."
 		    (setq current nil)
 		    (when exit-after-next (setq exit-after-next 'now)))
 		   ((= c ?\t)
-		    (condition-case nil
-			(setq tg (completing-read
-				  "Tag: "
-				  (or buffer-tags
-				      (with-current-buffer buf
-					(setq buffer-tags
-					      (org-get-buffer-tags))))))
-		      (quit (setq tg "")))
+                    (condition-case nil
+                        (unless tab-tags
+                          (setq tab-tags
+                                (delq nil
+                                      (mapcar (lambda (x)
+                                                (let ((item (car-safe x)))
+                                                  (and (stringp item)
+                                                       (list item))))
+                                              (org--tag-add-to-alist
+                                               (with-current-buffer buf
+                                                 (org-get-buffer-tags))
+                                               table))))))
+                    (setq tg (completing-read "Tag: " tab-tags))
 		    (when (string-match "\\S-" tg)
-		      (cl-pushnew (list tg) buffer-tags :test #'equal)
+		      (cl-pushnew (list tg) tab-tags :test #'equal)
 		      (if (member tg current)
 			  (setq current (delete tg current))
 			(push tg current)))