fsck: only provide oid/type in fsck_error callback

fsck: only provide oid/type in fsck_error callback

None of the callbacks actually care about having a "struct object";
they're happy with just the oid and type information. So let's give
ourselves more flexibility to avoid having a "struct object" by just
passing the broken-down fields.

Note that the callback already takes a "type" field for the fsck message
type. We'll rename that to "msg_type" (and use "object_type" for the
object type) to make the distinction explicit.

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