object-store.h: teach for_each_packed_object to ignore kept packs

object-store.h: teach for_each_packed_object to ignore kept packs

The next patch will reimplement a function that wants to iterate over
packed objects while ignoring packs which are marked as kept (either
in-core or on-disk).

Teach for_each_packed_object() to ignore all objects from those packs by
adding a new flag for each of the "kept" states that a pack can be in.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>