Re: [PATCH 5/8] midx: load multi-pack indices via their source

> To load a multi-pack index the caller is expected to pass both the
> repository and the object directory where the multi-pack index is
> located. While this works, this layout has a couple of downsides:
>
>   - We need to pass in information reduntant with the owning source,
>     namely its object directory and whether the source is local or not.
>
>   - We don't have access to the source when loading the multi-pack
>     index. If we had that access, we could store a pointer to the owning
>     source in the MIDX and thus deduplicate some information.
>
>   - Multi-pack indices are inherently specific to the object source and
>     its format. With the goal of pluggable object backends in mind we
>     will eventually want the backends to own the logic of reading and
>     writing multi-pack indices. Making the logic work on top of object
>     sources is a step into that direction.
>
> Refactor loading of multi-pack indices accordingly.
>
> This surfaces one small problem though: git-multi-pack-index(1) and our
> MIDX test helper both know to read and write multi-pack-indices located
> in a different object directory. This issue is addressed by adding the
> user-provided object directory as an in-memory alternate.
>