We don't actually have a Git-object collision, so the best
we can do is to run one of the shattered PDFs through
test-sha1. This should trigger the collision check and die.
In a sense this isn't really checking anything that the
upstream sha1collisiondetection project doesn't cover
already. But it at least makes sure that our build correctly
uses the library.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>