describe: pass oid struct by const pointer

describe: pass oid struct by const pointer

We pass a "struct object_id" to describe_blob() by value. This isn't
wrong, as an oid is composed only of copy-able values. But it's unusual;
typically we pass structs by const pointer, including object_ids. Let's
do so.

It similarly makes sense for us to hold that pointer in the callback
data (rather than yet another copy of the oid).

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