Re: [PATCH 6/8] midx-write.c: support reading an existing MIDX with `packs_to_include`

> Avoid unconditionally copying all packs from an existing MIDX into a new
> MIDX by checking that packs added via `fill_packs_from_midx()` don't
> appear in the `to_include` set, if one was provided.
> 
> Do so by calling `should_include_pack()` from both `add_pack_to_midx()`
> and `fill_packs_from_midx()`.

> diff --git a/midx-write.c b/midx-write.c
> index 9712ac044f..36ac4ab65b 100644
> --- a/midx-write.c
> +++ b/midx-write.c
> @@ -101,27 +101,13 @@ struct write_midx_context {
>  };
>  
>  static int should_include_pack(const struct write_midx_context *ctx,
> -			       const char *file_name)
> +			       const char *file_name,
> +			       int exclude_from_midx)
>  {
> -	/*
> -	 * Note that at most one of ctx->m and ctx->to_include are set,
> -	 * so we are testing midx_contains_pack() and
> -	 * string_list_has_string() independently (guarded by the
> -	 * appropriate NULL checks).
> -	 *
> -	 * We could support passing to_include while reusing an existing
> -	 * MIDX, but don't currently since the reuse process drags
> -	 * forward all packs from an existing MIDX (without checking
> -	 * whether or not they appear in the to_include list).
> -	 *
> -	 * If we added support for that, these next two conditional
> -	 * should be performed independently (likely checking
> -	 * to_include before the existing MIDX).
> -	 */
> -	if (ctx->m && midx_contains_pack(ctx->m, file_name))
> +	if (exclude_from_midx && ctx->m && midx_contains_pack(ctx->m, file_name))
>  		return 0;
> -	else if (ctx->to_include &&
> -		 !string_list_has_string(ctx->to_include, file_name))
> +	if (ctx->to_include && !string_list_has_string(ctx->to_include,
> +						       file_name))