bug#63470: [PATCH] Use faster option for running vc-hg status

[Eli Zaretskii <eliz@gnu.org>]

> From: Spencer Baugh <sbaugh@janestreet.com>
> Cc: Dmitry Gutov <dmitry@gutov.dev>,  63470@debbugs.gnu.org
> Date: Tue, 16 May 2023 16:39:49 -0400
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> Date: Fri, 12 May 2023 23:10:05 +0300
> >> Cc: 63470@debbugs.gnu.org
> >> From: Dmitry Gutov <dmitry@gutov.dev>
> >> 
> >> On 12/05/2023 22:57, Spencer Baugh wrote:
> >> > 
> >> > I could add a user option to revert to the old behavior, if you want.
> >> 
> >> We could cache it like we do with vc-git--program-version. That's a 
> >> simple memoization that doesn't take the host into account (though that 
> >> could be implemented, too).
> >
> > Yes, that'd be a good-enough solution.
> >
> >> But it'd really make things easier if we're just allowed to rely on some 
> >> new enough versions of Git and Hg.
> >
> > It isn't easy to be backward-compatible, but we should strive at doing
> > that.
> 
> OK, revised backwards-compatible patch attached.

Thanks, but I'd prefer to call the new function only once, and record
the result in some variable.  We do such things in umpteen other
places, so it looks strange to test the version each time only in this
case.

If you are afraid that somehow the version could change while the
Emacs session runs, we could add a command to recompute the version.
I think it's reasonable to ask the user to do this by hand, since
installing a new version of Mercurial should be something users are
aware of.