bug#51830: 29.0.50; tab-line-mode will override (setq-default tab-line-format "xxx")

[Eli Zaretskii]

> From: Juri Linkov <juri@linkov.net>
> Date: Sun, 14 Nov 2021 19:40:28 +0200
> Cc: 51830@debbugs.gnu.org, Feng Shu <tumashu@163.com>
> 
> > Yes, tab-line-mode resets the variable unconditionally:
> >
> > (define-minor-mode tab-line-mode
> >   "Toggle display of tab line in the windows displaying the current buffer."
> >   :lighter nil
> >   (setq tab-line-format (when tab-line-mode '(:eval (tab-line-format)))))
> >
> > Perhaps that should be done only when the variable's value is nil?
> 
> This needs also to keep the original value while disabling the mode.
> Do you think this should be installed in Emacs 28?

We had that code since Emacs 27.  Is it really urgent to fix that in
Emacs 28?