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

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>