t: regression git needs safe.directory when using sudo

Originally reported after release of v2.35.2 (and other maint branches)
for CVE-2022-24765 and blocking otherwise harmless commands that were
done using sudo in a repository that was owned by the user.

Add a new test script with very basic support to allow running git
commands through sudo, so a reproduction could be implemented and that
uses only `git status` as a proxy of the issue reported.

Note that because of the way sudo interacts with the system, a much
more complete integration with the test framework will require a lot
more work and that was therefore intentionally punted for now.

The current implementation requires the execution of a special cleanup
function which should always be kept as the last "test" or otherwise
the standard cleanup functions will fail because they can't remove
the root owned directories that are used.  This also means that if
failures are found while running, the specifics of the failure might
not be kept for further debugging and if the test was interrupted, it
will be necessary to clean the working directory manually before
restarting by running:

  $ sudo rm -rf trash\ directory.t0034-root-safe-directory/

The test file also uses at least one initial "setup" test that creates
a parallel execution directory under the "root" sub directory, which
should be used as top level directory for all repositories that are
used in this test file.  Unlike all other tests the repository provided
by the test framework should go unused.

Special care should be taken when invoking commands through sudo, since
the environment is otherwise independent from what the test framework
setup and might have changed the values for HOME, SHELL and dropped
several relevant environment variables for your test.  Indeed `git status`
was used as a proxy because it doesn't even require commits in the
repository to work and usually doesn't require much from the environment
to run, but a future patch will add calls to `git init` and that will
fail to honor the default branch name, unless that setting is NOT
provided through an environment variable (which means even a CI run
could fail that test if enabled incorrectly).

A new SUDO prerequisite is provided that does some sanity checking
to make sure the sudo command that will be used allows for passwordless
execution as root without restrictions and doesn't mess with git's
execution path.  This matches what is provided by the macOS agents that
are used as part of GitHub actions and probably nowhere else.

Most of those characteristics make this test mostly only suitable for
CI, but it might be executed locally if special care is taken to provide
for all of them in the local configuration and maybe making use of the
sudo credential cache by first invoking sudo, entering your password if
needed, and then invoking the test with:

  $ GIT_TEST_ALLOW_SUDO=YES ./t0034-root-safe-directory.sh

If it fails to run, then it means your local setup wouldn't work for the
test because of the configuration sudo has or other system settings, and
things that might help are to comment out sudo's secure_path config, and
make sure that the account you are using has no restrictions on the
commands it can run through sudo, just like is provided for the user in
the CI.

For example (assuming a username of marta for you) something probably
similar to the following entry in your /etc/sudoers (or equivalent) file:

  marta	ALL=(ALL:ALL) NOPASSWD: ALL

Reported-by: SZEDER Gábor <szeder.dev@gmail.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>

t/t0034-root-safe-directory.sh

@@ -0,0 +1,44 @@
+#!/bin/sh
+
+test_description='verify safe.directory checks while running as root'
+
+. ./test-lib.sh
+
+if [ "$GIT_TEST_ALLOW_SUDO" != "YES" ]
+then
+	skip_all="You must set env var GIT_TEST_ALLOW_SUDO=YES in order to run this test"
+	test_done
+fi
+
+test_lazy_prereq SUDO '
+	sudo -n id -u >u &&
+	id -u root >r &&
+	test_cmp u r &&
+	command -v git >u &&
+	sudo command -v git >r &&
+	test_cmp u r
+'
+
+test_expect_success SUDO 'setup' '
+	sudo rm -rf root &&
+	mkdir -p root/r &&
+	(
+		cd root/r &&
+		git init
+	)
+'
+
+test_expect_failure SUDO 'sudo git status as original owner' '
+	(
+		cd root/r &&
+		git status &&
+		sudo git status
+	)
+'
+
+# this MUST be always the last test
+test_expect_success SUDO 'cleanup' '
+	sudo rm -rf root
+'
+
+test_done