If we are in a git directory, get_git_work_tree() can return NULL.
While trying to determine whether or not the given paths are outside
the work tree, the following command would read from it anyways and
trigger a segmentation fault.
git diff / /
Signed-off-by: Clemens Buchacher <firstname.lastname@example.org>
Signed-off-by: Junio C Hamano <email@example.com>