object-name: backend-generic `repo_collect_ambiguous()`

The function `repo_collect_ambiguous()` is responsible for collecting objects whose IDs match a specific prefix. The information is then used to inform the user about which objects they could have meant in case a short object ID is ambiguous. The logic to do this uses the object disambiguation infrastructure and calls into backend-specific functions to iterate through loose and packed objects. This isn't really required anymore though: all we want to do is to enumerate objects that have such a prefix and then append those objects to a `struct oid_array`. This can be trivially achieved in a generic way now that `odb_for_each_object()` has learned to yield only objects that match such a prefix. Refactor the code to use the backend-generic infrastructure instead. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>