Introduce "sparse checkout"

Introduce "sparse checkout"

With skip-worktree bit, you can manually set it to unwanted files,
then remove them: you would have the so-called sparse checkout. The
disadvantages are:

 - Porcelain tools are not aware of this. Everytime you do an
   operation that may update working directory, skip-worktree may be
   cleared out. You have to set them again.

 - You still have to remove skip-worktree'd files manually, which is
   boring and ineffective.

These will be addressed in the following patches. This patch gives an
idea what is "sparse checkout" in Documentation/git-read-tree.txt.
This file is chosen instead of git-checkout.txt because it is quite
technical and user-unfriendly. I'd expect git-checkout.txt to have
something when Porcelain support is done.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>