t1509: fix failing "root work tree" test due to owner-check

When 8959555cee (setup_git_directory(): add an owner check for the top-level directory, 2022-03-02) tightened security surrounding directory ownership, it neglected to adjust t1509-root-work-tree.sh to take the new restriction into account. As a result, since the root directory `/` is typically not owned by the user running the test (indeed, t1509 refuses to run as `root`), the ownership check added by 8959555cee kicks in and causes the test to fail: fatal: detected dubious ownership in repository at '/' To add an exception for this directory, call: git config --global --add safe.directory / This problem went unnoticed for so long because t1509 is rarely run since it requires setting up a `chroot` environment or a sacrificial virtual machine in which `/` can be made writable and polluted by any user. Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>