contrib: remote-helpers: add move warnings (v2.0)

The tools are now maintained out-of-tree, and they have a regression
in v2.0. It's better to start warning the users as soon as possible.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
remote-helpers: point at their upstream repositories

Signed-off-by: Junio C Hamano <gitster@pobox.com>