Re: [PATCH 4/8] midx-write.c: extract `should_include_pack()`

> The add_pack_to_midx() callback used via for_each_file_in_pack_dir() is
> used to add packs with .idx files to the MIDX being written.
> 
> Within this function, we have a pair of checks that discards packs
> which:
> 
>   - appear in an existing MIDX, if we successfully read an existing MIDX
>     from disk
> 
>   - or, appear in the "to_include" list, if invoking the MIDX write
>     machinery with the `--stdin-packs` command-line argument.
> 
> In a future commit will want to call a slight variant of these checks