receive-pack: use find_commit_header() in check_nonce()

receive-pack: use find_commit_header() in check_nonce()

Use the public function find_commit_header() and remove find_header(),
as it becomes unused.  This is safe and appropriate because we pass the
NUL-terminated payload buffer to check_nonce() instead of its start and
length.  The underlying strbuf push_cert cannot contain NULs, as it is
built using strbuf_addstr(), only.

We no longer need to call strlen(), as find_commit_header() returns the
length of nonce already.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>