untracked cache: make a wrapper around {open,read,close}dir()

This allows us to feed different info to read_directory_recursive()
based on untracked cache in the next patch.

Helped-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>