Re: [PATCH v2 4/4] midx-write.c: use `--stdin-packs` when repacking

> When constructing a new pack `git multi-pack-index repack` provides a
> list of objects which is the union of objects in all MIDX'd packs which
> were "included" in the repack.
>
> Though correct, this typically yields a poorly structured pack, since
> providing the objects list over stdin does not give pack-objects a
> chance to discover the namehash values for each object, leading to
> sub-optimal delta selection.
>
> We can use `--stdin-packs` instead, which has a couple of benefits:
>
>   - it does a supplemental walk over objects in the supplied list of
>     packs to discover their namehash, leading to higher-quality delta
>     selection
>
>   - it requires us to list far less data over stdin; instead of listing
>     each object in the resulting pack, we need only list the
>     constituent packs from which those objects were selected in the MIDX
>
> Of course, this comes at a slight cost: though we save time on listing
> packs versus objects over stdin[^1] (around ~650 milliseconds), we add a
> non-trivial amount of time walking over the given objects in order to
> find better deltas.
>
> In general, this is likely to more closely match the user's expectations
> (i.e. that packs generated via `git multi-pack-index repack` are written
> with high-quality deltas). But if not, we can always introduce a new
> option in pack-objects to disable the supplemental object walk, which
> would yield a pure CPU-time savings, at the cost of the on-disk size of
> the resulting pack.
>
> [^1]: In a patched version of Git that doesn't perform the supplemental
>   object walk in `pack-objects --stdin-packs`, we save around ~650ms
>   (from 5.968 to 5.325 seconds) when running `git multi-pack-index
>   repack --batch-size=0` on git.git with all objects packed, and all
>   packs in a MIDX.

> -	strvec_push(&cmd.args, "pack-objects");
> +	strvec_pushl(&cmd.args, "pack-objects", "--stdin-packs", "--non-empty",
> +		     NULL);
>  
>  	strvec_pushf(&cmd.args, "%s/pack/pack", object_dir);
>  
> @@ -1498,16 +1499,15 @@ int midx_repack(struct repository *r, const char *object_dir, size_t batch_size,
>  	}
>  
>  	cmd_in = xfdopen(cmd.in, "w");
> -
> -	for (i = 0; i < m->num_objects; i++) {
> -		struct object_id oid;
> -		uint32_t pack_int_id = nth_midxed_pack_int_id(m, i);
> -
> -		if (!include_pack[pack_int_id])
> +	for (i = 0; i < m->num_packs; i++) {
> +		struct packed_git *p = m->packs[i];
> +		if (!p)
>  			continue;
>  
> -		nth_midxed_object_oid(&oid, m, i);
> -		fprintf(cmd_in, "%s\n", oid_to_hex(&oid));
> +		if (include_pack[i])
> +			fprintf(cmd_in, "%s\n", pack_basename(p));
> +		else
> +			fprintf(cmd_in, "^%s\n", pack_basename(p));
>  	}
>  	fclose(cmd_in);