midx: avoid negative array index

nth_midxed_pack_int_id() returns the index of the pack file in the multi
pack index's list of packfiles that the specified object. The index is
returned as a uint32_t. Storing this in an int will make the index
negative if the most significant bit is set. Fix this by using uint32_t
as the rest of the code does. This is unlikely to be a practical problem
as it requires the multipack index to reference 2^31 packfiles.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: phillipwood

midx-write.c

@@ -1566,7 +1566,7 @@ int expire_midx_packs(struct repository *r, const char *object_dir, unsigned fla
 					  _("Counting referenced objects"),
 					  m->num_objects);
 	for (i = 0; i < m->num_objects; i++) {
-		int pack_int_id = nth_midxed_pack_int_id(m, i);
+		uint32_t pack_int_id = nth_midxed_pack_int_id(m, i);
 		count[pack_int_id]++;
 		display_progress(progress, i + 1);
 	}
@@ -1697,7 +1697,7 @@ static void fill_included_packs_batch(struct repository *r,
 
 	total_size = 0;
 	for (i = 0; total_size < batch_size && i < m->num_packs; i++) {
-		int pack_int_id = pack_info[i].pack_int_id;
+		uint32_t pack_int_id = pack_info[i].pack_int_id;
 		struct packed_git *p = m->packs[pack_int_id];
 		uint64_t expected_size;