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

[Juri Linkov]

>> It doesn't use 'region-history'.  I meant using 'region-history-mode'
>> on the output buffer.
>
> 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.

Maybe region-history-mode is not the best name, but at least
it provides fontification for revisions composed of both
the commit metadata and the diff of changes.

> The reason I think this should be a vc-diff subcommand is that most
> kinds of VCS describe a revision as diffs.  Where a revision's
> description is expected to include meta-data, i.e. the VCS backend
> provides a command to show a revision in that format, we should use
> that backend command, of course.  But conceptually we just show diffs,
> so inventing a whole new class of VC commands for a minor variant of
> diffs display sounds sub-optimal to me.

Similar to wave-particle duality, in revision's duality the revision
may be described as either a log metadata or a diff depending on the
observer's point of view.  A new separate command resolves such dilemma.

> Also, why only implement this for a single backend?  The corresponding
> diff commands exist in every VCS we support, so let's implement this
> for all of them, okay?  Let us know if you need help with other VCSes.

A new command was intended to produce the output similar to e.g.

http://git.savannah.gnu.org/cgit/emacs.git/commit/?id=6f30642973975a317a9c94ceba737a4bafc89919

i.e. first comes the metadata, then the commit message, and finally the diff.

Do other VCSes or their interfaces produce a similar layout?