Without this, attempting to index a pack containing objects that have been
replaced results in a fatal error that looks like:
fatal: SHA1 COLLISION FOUND WITH <replaced-object> !
Signed-off-by: Nelson Elhage <nelhage@ksplice.com> Acked-by: Christian Couder <chriscool@tuxfamily.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>