archive tests: use a cheaper "zipinfo -h" invocation to get header

archive tests: use a cheaper "zipinfo -h" invocation to get header

Change an invocation of zipinfo added in 19ee29401d (t5004: test ZIP
archives with many entries, 2015-08-22) to simply ask zipinfo for the
header info, rather than spewing out info about the entire archive and
race to kill it with SIGPIPE due to the downstream "head -2".

I ran across this because I'm adding a "set -o pipefail" test
mode. This won't be needed for the version of the mode that I'm
introducing (which currently relies on a patch to GNU bash), but I
think this is a good idea anyway.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>