bug#64055: [WIP Patch] Enable editing commit messages - vc-git-modify-change-comment

[Eli Zaretskii <eliz@gnu.org>]

> From: Sean Whitton <spwhitton@spwhitton.name>
> Cc: 64055@debbugs.gnu.org,  Morgan.J.Smith@outlook.com,  rpluim@gmail.com,
>   dmitry@gutov.dev
> Date: Sun, 20 Oct 2024 08:56:12 +0800
> 
> On the other hand, the design principle doesn't apply directly to the
> new get-change-comment backend action, because src, rcs, sccs, cvs and
> svn do not *require* an implementation of get-change-comment in order
> for the log-view-modify-change-comment command to work, so far as I
> understand it -- otherwise the feature would not have been implemented
> the way it is, or we'd have bug reports.
> 
> Git, I think, does require an implementation of get-change-comment, or
> at least, I concluded that it's the most natural way to implement
> log-view-modify-change-comment for git.

It is this difference between Git and the rest that I don't yet see,
and you didn't explain it, just stated its existence.  Why is the
method necessary for Git, but not for other VCSes? they all produce
similar displays of log messages, complete with Author, Date, etc.,
and so the difficulties of extracting just the change log message
should affect all of them, no?