bug#62940: 29.0.60; vc: no easy way to get diff of all outgoing changes

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 62940@debbugs.gnu.org, Filipp Gunbin <fgunbin@fastmail.fm>
> Date: Sat, 14 Sep 2024 05:11:48 +0300
> From: Dmitry Gutov <dmitry@gutov.dev>
> 
> > (vc-git-log-outgoing): Use vc-git-pushed-revision.
> > * lisp/vc/vc-hg.el (vc-hg-pushed-revision): Add.
> > * lisp/vc/vc.el (vc-default-pushed-revision): Add new backend function
> > with default implementation returning nil.
> 
> Am I too used to Git, or should this be called upstream-revision or 
> origin-revision?

upstream-revision is definitely better than
vc-default-pushed-revision, since some VCSes don't even have the
"push" operation.

More importantly, this change must be accompanied with a suitable
update of the user manual, where we should explain what commit is
suggested as the default.  "Last pushed revision" is somewhat vague
and inaccurate, because the user could switch branches or remotes, or
do something else.  We should find a more accurate description.  Also,
the doc string of vc-root-diff needs to be updated with this
information.

> Maybe we would prefer to keep the current suggestion as the first 
> default, and have the upstream as the second suggestion (on M-n)? For 
> backward compatibility anyway.

Sounds like a good idea to me.