bug#38044: 27.0.50; There should be an easier way to look at a specific vc commit

[Eli Zaretskii]

> From: Juri Linkov <juri@linkov.net>
> Cc: Eli Zaretskii <eliz@gnu.org>,  larsi@gnus.org,  stephen.berman@gmx.net,
>   38044@debbugs.gnu.org
> Date: Sun, 24 Nov 2019 00:43:37 +0200
> 
> @@ -2416,7 +2416,10 @@ vc-log-internal-common
>        (let ((inhibit-read-only t))
>       ;; log-view-mode used to be called with inhibit-read-only bound
>       ;; to t, so let's keep doing it, just in case.
> -     (vc-call-backend backend 'log-view-mode)
> +     (vc-call-backend backend
> +                         (if (eq type 'with-diff)
> +                             'region-history-mode
> +                           'log-view-mode))

Can we avoid calling region-history-mode if the backend doesn't
implement it?  Or do you plan on adding that to all backends?

Thanks.