Rework +dired/insert-all-subdirectories

If argument is provided (C-u), +dired/insert-all-subdirectories
recursively inserts subdirectories until there are no subdirectories
left.  Otherwise, perform the same behaviour as before i.e. only
insert subdirectories currently present in the buffer.

This allows dired to have a "treelike" view without having to use an
external library.

Emacs/.config/emacs/config.org

@@ -3236,12 +3236,27 @@ Here I setup dired with a few niceties
                         '("\\.webm\\'"   "mpv")
                         '("\\.mp[34]\\'" "mpv")
                         '("\\.mkv\\'"    "mpv"))
-  (defun +dired/insert-all-subdirectories ()
-    "Insert all subdirectories currently viewable."
-    (interactive)
+
+  (defun +dired/--subdirs-not-inserted ()
+    (dired-unmark-all-marks)
     (dired-mark-directories nil)
-    (mapc #'dired-insert-subdir (dired-get-marked-files))
-    (dired-unmark-all-marks)))
+    (let* ((subdirs-present (mapcar #'car dired-subdir-alist))
+           (subdirs-left (thread-last
+                           (dired-get-marked-files)
+                           (mapcar #'(lambda (x) (concat x "/")))
+                           (cl-remove-if #'(lambda (f) (member f subdirs-present))))))
+      (dired-unmark-all-marks)
+      subdirs-left))
+
+  (defun +dired/insert-all-subdirectories (&optional arg)
+    "Insert all subdirectories recursively."
+    (interactive "P")
+    (let ((subdirs-left (+dired/--subdirs-not-inserted)))
+      (if (null arg)
+          (mapc #'dired-insert-subdir subdirs-left)
+        (while subdirs-left
+          (mapc #'dired-insert-subdir subdirs-left)
+          (setq subdirs-left (+dired/--subdirs-not-inserted)))))))
 #+end_src
 *** image-dired
 Image dired is a little cherry on top for Dired: the ability to look