odb: get rid of `the_repository` when handling the primary source

odb: get rid of `the_repository` when handling the primary source

The functions `set_temporary_primary_odb()` and `restore_primary_odb()`
are responsible for managing a temporary primary source for the
database. Both of these functions implicitly rely on `the_repository`.

Refactor them to instead take an explicit object database parameter as
argument and adjust callers. Rename the functions accordingly.

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