odb/source-loose: stub out remaining callbacks

odb/source-loose: stub out remaining callbacks

Stub out remaining callback functions for the "loose" backend.

Note that we also stub out transactions for loose objects. In fact, we
already have the infrastructure in place for those, and we could in
theory implement those, as well. But there are separate efforts ongoing
to polish up transactional interfaces, and doing so now would likely
result in some messiness. This omission will thus be worked on in a
subsequent patch series, once the dust has settled.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>