Re: [PATCH 3/8] midx-write.c: pass `start_pack` to `get_sorted_entries()`

> The function `get_sorted_entries()` is broadly responsible for
> building an array of the objects to be written into a MIDX based on the
> provided list of packs.
> 
> If we have loaded an existing MIDX, however, we may not use all of its
> packs, despite loading them into the ctx->info array.
> 
> The existing implementation simply skips past the first
> ctx->m->num_packs (if ctx->m is non-NULL, indicating that we loaded an
> existing MIDX). Future changes (outside the scope of this patch series)
> to the MIDX code will require us to skip *at most* that number[^1].
> 
> We could tag each pack with a bit that indicates the pack's contents
> should be included in the MIDX. But we can just as easily determine the
> number of packs to skip by passing in the number of packs we learned
> about after processing an existing MIDX.