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

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

The function `force_object_loose()` forces an object to become a loose
object in case it only exists in its packed form. To do so it implicitly
relies on `the_repository`.

Refactor the function by passing a `struct odb_source` as parameter.
While the check whether any such loose object exists already acts on the
whole object database, writing the loose object happens in one specific
source.

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