http: use struct object_id instead of bare sha1

http: use struct object_id instead of bare sha1

The dumb-http walker code still passes around and stores object ids as
"unsigned char *sha1". Let's modernize it.

There's probably still more work to be done to handle dumb-http fetches
with a new, larger hash. But that can wait; this is enough that we can
now convert some of the low-level object routines that we call into from
here (and in fact, some of the "oid.hash" references added here will be
further improved in the next patch).

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