Re: [PATCH 01/10] packfile: introduce 'find_kept_pack_entry()'

> So the more compelling argument, I think, is something like:
>
>   - you sometimes want to know if object X is any kept packs
>
>   - you can't use find_pack_entry(), because it only gives you the first
>     pack it finds
>
>   - you can walk over all packs and look for the object in each.
>     pack-objects does this. But it's slow, because you are looking in
>     packs you don't care about.
>
>   - so it's helpful for the lookup to know up front which packs are
>     interesting to find objects in and which are not, to avoid looking
>     in the uninteresting ones