Really protect against unwanted indentation after undo

When I am undoing changes to a region I really want the changes to be
undone verbatim, that is, I want the buffer's contents to be like they
were before the change that I want to undo occured.  This is the very
definition of undo.

It is probably the rationale for have undo-in-progress in
aggressive-indent--internal-dont-indent-if in the first place. It
means that the undo command won't immediately cause for reindentation.

But it doesn't fix the whole problem, because changes performed by
undo itself are still recorded into aggressive-indent--changed-list
and the very next command, be it a move or an edit somewhere else,
will still indent those regions.  Among other things, it's impossible
in practice to use `undo' to undo an aggressive indent of a region.

The proposed fix checks undo-in-progress before registering a change.
It's possible that other elements (but maybe not all) in
aggressive-indent--internal-dont-indent-if merit this treatment, too.

* aggressive-indent.el (aggressive-indent--keep-track-of-changes):
Check undo-in-progress.

aggressive-indent.el

@@ -426,7 +426,8 @@ typing, try tweaking this number."
 (defun aggressive-indent--keep-track-of-changes (l r &rest _)
   "Store the limits (L and R) of each change in the buffer."
   (when aggressive-indent-mode
-    (push (list l r) aggressive-indent--changed-list)
+    (unless undo-in-progress
+      (push (list l r) aggressive-indent--changed-list))
     (unless (timerp aggressive-indent--idle-timer)
       (setq aggressive-indent--idle-timer
             (run-with-idle-timer aggressive-indent-sit-for-time t #'aggressive-indent--indent-if-changed)))))