connected: search promisor objects generically

connected: search promisor objects generically

When performing connectivity checks we have to figure out whether any of
the new objects are promisor objects, as we cannot assume full
connectivity if so.

This check is performed by iterating through all packfiles in the
repository and searching each of them for the given object. Of course,
this mechanism is quite specific to implementation details of the object
database, as we assume that it uses packfiles in the first place.

Refactor the logic so that we instead use `odb_for_each_object_ext()`
with an object prefix filter and the `ODB_FOR_EACH_OBJECT_PROMISOR_ONLY`
flag. This will yield all objects that have the exact object name and
that are part of a promisor pack in a generic way.

Add a test to verify that we indeed use the optimization.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>