Add a support library that provides one function that can be used
to run a "scriplet" of commands through sudo and that helps invoking
sudo in the slightly awkward way that is required to ensure it doesn't
block the call (if shell was allowed as tested in the prerequisite)
and it doesn't run the command through a different shell than the one
we intended.

Add additional negative tests as suggested by Junio and that use a
new workspace that is owned by root.

Document a regression that was introduced by previous commits where
root won't be able anymore to access directories they own unless
SUDO_UID is removed from their environment.

The tests document additional ways that this new restriction could
be worked around and the documentation explains why it might be instead
considered a feature, but a "fix" is planned for a future change.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Phillip Wood <phillip.wood123@gmail.com>
Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>