bug#64989: 29.1; Comment or uncomment lines in elisp-mode will show message in minibuffer

[Eli Zaretskii <eliz@gnu.org>]

> From: Eason Huang <aqua0210@foxmail.com>
> Date: Tue, 01 Aug 2023 19:13:59 +0800
> 
> Hello Emacs,
> 
> When update to the new released Emacs 29.1, I found that when comment or
> uncomment lines in emacs-lisp-mode will show message in minibuffer as
> below:
> ```
> combine-change-calls: buffer-undo-list broken
> ```
> Steps to reproduce:
> 1. create a test.el file with any Elisp code. such as:
> ```
> (message "Test")
> ```
> 2. start emacs and open the test.el file with `emacs -Q ~/test.el`
> 3. Use C-x C-; (comment-line) to comment the code in test.el
> 4. Now you will see the message mentioned above.

Thanks.

Stefan, I think this is due to the commit below:

| Author:     Stefan Monnier <monnier@iro.umontreal.ca>
| AuthorDate: Fri Jan 13 17:38:04 2023 -0500
| Commit:     Stefan Monnier <monnier@iro.umontreal.ca>
| CommitDate: Fri Jan 13 17:38:04 2023 -0500
|
|     * lisp/subr.el (combine-change-calls-1): Fix bug#60467
|
|     Don't stop at timestamps.  Strip them for now, to be on the safe side.
|     Don't merge into `master` where we'll use a better fix.

My "partial bisecting" indicates that the bug was introduced between
Dec 31, 2022 and Jan 31, 2023, and the above is the only commit
in-between that could matter.

If indeed this is the culprit, can we perhaps backport the "better
fix" from master to the emacs-29 branch?