cache-tree.c: use ALLOC_GROW() in find_subtree()

Signed-off-by: Dmitry S. Dolzhenko <dmitrys.dolzhenko@yandex.ru>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache-tree.c

@@ -75,11 +75,7 @@ static struct cache_tree_sub *find_subtree(struct cache_tree *it,
 		return NULL;
 
 	pos = -pos-1;
-	if (it->subtree_alloc <= it->subtree_nr) {
-		it->subtree_alloc = alloc_nr(it->subtree_alloc);
-		it->down = xrealloc(it->down, it->subtree_alloc *
-				    sizeof(*it->down));
-	}
+	ALLOC_GROW(it->down, it->subtree_nr + 1, it->subtree_alloc);
 	it->subtree_nr++;
 
 	down = xmalloc(sizeof(*down) + pathlen + 1);