git{,-blame}.el: remove old bitrotting Emacs code

git{,-blame}.el: remove old bitrotting Emacs code

The git-blame.el mode has been superseded by Emacs's own
vc-annotate (invoked by C-x v g). Users of the git.el mode are now
much better off using either Magit or the Git backend for Emacs's own
VC mode.

These modes were added over 10 years ago when Emacs's own Git support
was much less mature, and there weren't other mature modes in the wild
or shipped with Emacs itself.

These days these modes have few if any users, and users of git aren't
well served by us shipping these (some OS's install them alongside git
by default, which is confusing and leads users astray).

So let's remove these per Alexandre Julliard's message to the
ML[1]. If someone still wants these for some reason they're better
served by hosting these elsewhere (e.g. on ELPA), instead of us
distributing them with git.

However, since downstream packagers such as Debian are packaging this
as git-el it's less disruptive to still carry these files as Elisp
code that'll error out with a message suggesting alternatives, rather
than drop the files entirely[2].

Then rather than receive a cryptic load error when they upgrade
existing users will get an error directing them to the README file, or
to just stop requiring these modes. I think it makes sense to link to
GitHub's hosting of contrib/emacs/README (which'll be updated by the
time users see this) so they don't have to hunt down the packaged
README on their local system.

1. "Re: [PATCH] git.el: handle default excludesfile
   properly" (87muzlwhb0.fsf@winehq.org) --
   https://public-inbox.org/git/87muzlwhb0.fsf@winehq.org/

2. "Re: [PATCH v3] git{,-blame}.el: remove old bitrotting Emacs
   code" (20180327165751.GA4343@aiede.svl.corp.google.com) --
   https://public-inbox.org/git/20180327165751.GA4343@aiede.svl.corp.google.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>