treewide: enumerate promisor objects via `odb_for_each_object()`

treewide: enumerate promisor objects via `odb_for_each_object()`

We have multiple callsites where we enumerate all promisor objects in
the object database via `for_each_packed_object()`. This is done by
passing the `ODB_FOR_EACH_OBJECT_PROMISOR_ONLY` flag, which causes us to
skip over all non-promisor objects.

These callsites can be trivially converted to `odb_for_each_object()` as
we know to skip enumeration of loose objects in case the `PROMISOR_ONLY`
flag was passed by the caller.

Refactor the sites accordingly.

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