Add TEMPORARY command to migrate Org filetags

This is due to the changes we made to the format of the Org front matter
where filetags are now written in the standard colon-separated
notation (check commit 37dca91).

Thanks to Alan Schmitt for bringing this matter to my attention:
https://lists.sr.ht/~protesilaos/denote/%3C871qu0jw5l.fsf%40protesilaos.com%3E

denote.el

@@ -2204,5 +2204,48 @@ Consult the manual for template samples."
 
 (add-hook 'org-capture-after-finalize-hook #'denote-org-capture-delete-empty-file)
 
+;;;; For the migration of old Org filetags
+
+(defun denote--migrate-org-files ()
+  "Return list of Org files in variable `denote-directory'."
+  (seq-remove
+   (lambda (file)
+     (not (string= (file-name-extension file) "org")))
+   (denote--directory-files)))
+
+;;;###autoload
+(defun denote-migrate-old-org-filetags ()
+  "Rewrite Org filetags' value as colon-separated.
+
+Change the filetags from:
+
+    #+filetags:   one  two
+
+To the standard format of:
+
+    #+filetags:  :one:two:
+
+A single tags chnages from TAG to :TAG:.
+
+Denote used to format filetags with two spaces between them, but
+this is not fully supported by Org.  The colon-separated entries
+are the rule.
+
+The rewrite DOES NOT SAVE BUFFERS.  The user is expected to
+review the changes, such as by using `diff-buffer-with-file'.
+Multiple buffers can be saved with `save-some-buffers' (check its
+doc string).
+
+This command is provided for the convenience of the user.  It
+shall be deprecated and eventually removed from future versions
+of Denote.  Written on 2022-08-10 for version 0.5.0."
+  (interactive)
+  (when-let (((yes-or-no-p "Rewrite filetags in Org files to use colons (buffers are NOT saved)?"))
+             (files (denote--migrate-org-files)))
+    (dolist (file files)
+      (when-let* ((kw (denote--front-matter-keywords-to-list file))
+                  ((denote--edit-front-matter-p file)))
+        (denote--rewrite-keywords file kw)))))
+
 (provide 'denote)
 ;;; denote.el ends here