Re: [PATCH v2 02/19] midx: add new fields for incremental MIDX chains

> The incremental MIDX chain feature is designed around the idea of
> indexing into a concatenated lexicographic ordering of object IDs
> present in the MIDX.
> 
> When given an object position, the MIDX machinery needs to be able to
> locate both (a) which MIDX layer contains the given object, and (b) at
> what position *within that MIDX layer* that object appears.
> 
> To do this, three new fields are added to the `struct multi_pack_index`:
> 
>   - struct multi_pack_index *base_midx;
>   - uint32_t num_objects_in_base;
>   - uint32_t num_packs_in_base;
> 
> These three fields store the pieces of information suggested by their
> respective field names. In turn, the `num_objects_in_base` and
> `num_packs_in_base` fields are used to crawl backwards along the
> `base_midx` pointer to locate the appropriate position for a given
> object within the MIDX that contains it.