bug#36861: 27.0.50; display-fill-column-indicator-mode in log-edit-mode

[Eli Zaretskii <eliz@gnu.org>]

> From: Juri Linkov <juri@linkov.net>
> Date: Tue, 30 Jul 2019 23:44:53 +0300
> 
> It has an unpleasant effect when git commit abruptly fails when
> the length of the first line of the commit message is longer than
> 78 characters.  This patch enables a visual indication in the
> log-edit buffer to help not to exceed the limit beforehand:
> 
> diff --git a/.dir-locals.el b/.dir-locals.el
> index ffd65c8802..e56d8c753b 100644
> --- a/.dir-locals.el
> +++ b/.dir-locals.el
> @@ -10,7 +10,9 @@
>                 (electric-quote-comment . nil)
>                 (electric-quote-string . nil)))
>   (log-edit-mode . ((log-edit-font-lock-gnu-style . t)
> -                   (log-edit-setup-add-author . t)))
> +                   (log-edit-setup-add-author . t)
> +                   (fill-column . 78)
> +                   (eval . (display-fill-column-indicator-mode))))

This will cause an annoying message and prompt when editing Emacs
sources with an Emacs which doesn't yet have
display-fill-column-indicator-mode, right?  Can we avoid that?  I
routinely need to work on the latest sources with an older Emacs.