mktag: call "check_sha1_signature" with the replacement sha1

mktag: call "check_sha1_signature" with the replacement sha1

Otherwise we get a "sha1 mismatch" error for replaced objects.

Note that I am not sure at all that this is a good change.
It may be that we should just refuse to tag a replaced object. But
in this case we should probably give a meaningfull error message
instead of "sha1 mismatch".

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>