odb: trivial refactorings to get rid of `the_repository`

odb: trivial refactorings to get rid of `the_repository`

All of the external functions provided by the object database subsystem
don't depend on `the_repository` anymore, but some internal functions
still do. Refactor those cases by plumbing through the repository that
owns the object database.

This change allows us to get rid of the `USE_THE_REPOSITORY_VARIABLE`
preprocessor define.

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