object-file: get rid of `the_repository` in `has_loose_object()`

object-file: get rid of `the_repository` in `has_loose_object()`

We implicitly depend on `the_repository` in `has_loose_object()`.
Refactor the function to accept an `odb_source` as input that should be
checked for such a loose object.

This refactoring changes semantics of the function to not check the
whole object database for such a loose object anymore, but instead we
now only check that single source. Existing callers thus need to loop
through all sources manually now.

While this change may seem illogical at first, whether or not an object
exists in a specific format should be answered by the source using that
format. As such, we can eventually convert this into a generic function
`odb_source_has_object()` that simply checks whether a given object
exists in an object source. And as we will know about the format that
any given source uses it allows us to derive whether the object exists
in a given format.

This change also makes `has_loose_object_nonlocal()` obsolete. The only
caller of this function is adapted so that it skips the primary object
source.

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