midx-write.c: enumerate pack_int_id values directly

Our `midx-write.c::fill_packs_from_midx()` function currently enumerates
the range [0, m->num_packs), and then shifts its index variable up by
`m->num_packs_in_base` to produce a valid `pack_int_id`.

Instead, directly enumerate the range:

    [m->num_packs_in_base, m->num_packs_in_base + m->num_packs)

, which are the original pack_int_ids themselves as opposed to the
indexes of those packs relative to the MIDX layer they are contained
within.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: ttaylorr

midx-write.c

@@ -932,11 +932,11 @@ static int fill_packs_from_midx(struct write_midx_context *ctx)
 	for (m = ctx->m; m; m = m->base_midx) {
 		uint32_t i;
 
-		for (i = 0; i < m->num_packs; i++) {
+		for (i = m->num_packs_in_base;
+		     i < m->num_packs_in_base + m->num_packs; i++) {
 			ALLOC_GROW(ctx->info, ctx->nr + 1, ctx->alloc);
 
-			if (fill_pack_from_midx(&ctx->info[ctx->nr], m,
-						m->num_packs_in_base + i) < 0)
+			if (fill_pack_from_midx(&ctx->info[ctx->nr], m, i) < 0)
 				return -1;
 
 			ctx->nr++;