busybox's sed isn't binary clean.
Thus, triggers false-negative on this test.
We could replace sed with perl on this usecase.
But, we could slightly modify the helper to discard unwanted data in the
beginning.
Fix the false negative by updating this helper.
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>