Really protect against unwanted indentation after undo

When I am undoing changes to a region I really want the changes to be
undone verbatim.  This is probably the rationale for have
undo-in-progress in aggressive-indent--internal-dont-indent-if in the
first place.

But it doesn't fix the whole problem, because changes performed by
undo are still recorded into aggressive-indent--changed-list and the
very next command will indent those regions.  This means it's
impossible in practice to use `undo' to undo an aggressive indent of a
region.

The fix proposed here 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)))))