Add test for using Git at root of file system

Add test for using Git at root of file system

This kind of test requires a throw-away root filesystem so that it can
play on. If you have such a system, go ahead, "chmod 777 /" and run
this test manually. Because this is a dangerous test, you are required
to set an env variable, and not to use root to run it.

Script prepare-root.sh may help you set up a chroot environment with
Git test suite inside. You will need Linux, static linked busybox,
rsync and root permission to use it.

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