This patch introduces core.sparseCheckout, which will control whether
sparse checkout support is enabled in unpack_trees()

It also loads sparse-checkout file that will be used in the next patch.
I split it out so the next patch will be shorter, easier to read.
Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>