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

[Eli Zaretskii]

> From: Andrii Kolomoiets <andreyk.mad@gmail.com>
> Date: Fri, 22 Nov 2019 00:36:18 +0200
> Cc: 38044@debbugs.gnu.org
> 
> > Ah, okay.  Btw, region-history-mode is a Git-only feature, so I don't
> > think I understand why it is in vc.el and not in vc-git.el.
> 
> Experimental option 'line-range' to command 'log' is added in mercurial 4.4
> 'region-history' is also implemented in vc-hgcmd.

I know, see commit 1110d14.  But I was talking about
region-history-mode, not vc-region-history.  By placing that mode in
vc.el, we now require _every_ backend to implement this mode, which
doesn't make a lot of sense to me, and you can see in that commit what
that causes for backends that do want to implement region-history.