fsck: use oids rather than objects for object_name API

fsck: use oids rather than objects for object_name API

We don't actually care about having object structs; we only need to look
up decorations by oid. Let's accept this more limited form, which will
give our callers more flexibility.

Note that the decoration API we rely on uses object structs itself (even
though it only looks at their oids). We can solve this by switching to
a kh_oid_map (we could also use the hashmap oidmap, but it's more
awkward for the simple case of just storing a void pointer).

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