contrib/vim: simplify instructions for old vim support

contrib/vim: simplify instructions for old vim support

Rely on the upstream filetype.vim instead of duplicating its rules in
git's instructions for syntax highlighting support on pre-7.2 vim
versions.

The result is a shorter contrib/vim/README.  More importantly, it lets
us punt on maintenance of the autocmd rules.

So now when we fix the upstream gitsendemail rule in light of commit
eed6ca7, new git users stuck on old vim reading contrib/vim/README can
automagically get the fix without any further changes needed to git.

Once the world has moved on to vim 7.2+ completely, we can get rid of
these instructions, but for now if they are this simple it's
effortless to keep them.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>