cat-file: use type enum instead of buffer for -t option

cat-file: use type enum instead of buffer for -t option

Now that we no longer support OBJECT_INFO_ALLOW_UNKNOWN_TYPE, there is
no need to pass a strbuf into oid_object_info_extended() to record the
type. The regular object_type enum is sufficient to capture all of the
types we will allow.

This simplifies the code a bit, and will eventually let us drop
object_info's type_name strbuf support.

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