odb: add `source` field to struct object_info_source

odb: add `source` field to struct object_info_source

The previous commit introduced `struct object_info_source` as an opt-in
container for backend-specific information, but for now we only moved
preexisting data into this structure. Most importantly, the caller has
no way yet to learn about which source an object was actually looked up
from. Instead, callers have to rely on the `whence` enum to distinguish
the object type, but cannot use that enum to tell the object source.

Add a `struct odb_source *source` field to the structure and populate it
from each backend's lookup path.

The `whence` enum is still set and used by callers; it will be removed
in a subsequent commit now that `sourcep->source` can identify the
backend on its own.

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