fsck: accept an oid instead of a "struct tag" for fsck_tag()

fsck: accept an oid instead of a "struct tag" for fsck_tag()

We don't actually look at the tag struct in fsck_tag() beyond its oid
and type (which is obviously OBJ_TAG). Just taking an oid gives our
callers more flexibility to avoid creating or parsing a struct, and
makes it clear that we are fscking just what is in the buffer, not any
pre-parsed bits from the struct.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>