pack: use 64 bits for the number of objects

Keeping it as a 32-bit value means the min size calculation overflows or gets
truncated which can lead to issues with large packfiles.

src/libgit2/pack.c

@@ -200,7 +200,8 @@ static void pack_index_free(struct git_pack_file *p)
 static int pack_index_check_locked(const char *path, struct git_pack_file *p)
 {
 	struct git_pack_idx_header *hdr;
-	uint32_t version, nr, i, *index;
+	uint32_t version, i, *index;
+	uint64_t nr = 0;
 	void *idx_map;
 	size_t idx_size;
 	struct stat st;
@@ -246,7 +247,6 @@ static int pack_index_check_locked(const char *path, struct git_pack_file *p)
 		version = 1;
 	}
 
-	nr = 0;
 	index = idx_map;
 
 	if (version > 1)
@@ -287,8 +287,8 @@ static int pack_index_check_locked(const char *path, struct git_pack_file *p)
 		 * variable sized table containing 8-byte entries
 		 * for offsets larger than 2^31.
 		 */
-		unsigned long min_size = 8 + (4 * 256) + (nr * (p->oid_size + 4 + 4)) + (p->oid_size * 2);
-		unsigned long max_size = min_size;
+		uint64_t min_size = 8 + (4 * 256) + (nr * (p->oid_size + 4 + 4)) + (p->oid_size * 2);
+		uint64_t max_size = min_size;
 
 		if (nr)
 			max_size += (nr - 1)*8;