In the case of a corrupt repository, git ls-tree may report an error but
presently it exits with a code of 0.
This change uses the return code of read_tree_recursive instead.
Improved-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Jon Seymour <jon.seymour@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>