odb: return newly created in-memory sources

odb: return newly created in-memory sources

Callers have no trivial way to obtain the newly created object database
source when adding it to the in-memory list of alternates. While not yet
needed anywhere, a subsequent commit will want to obtain that pointer.

Refactor the function to return the source to make it easily accessible.

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