Suppose that you've just done a
git checkout -f to cleanup your local modifications.
New files added will slip out of the previous command, because they're treated by git as "untracked".
Here's a one-liner to get rid of these little fellas (requires a Ruby interpreter, because I <3 Ruby):
git status \ --untracked-files=all \ --no-column \ --short | \ rm $(ruby -e "puts STDIN.read.lines.map(&:split).map(&:last).join(' ')")
Update: as @kwstannard
rightfully pointed out in the comments, there's also the prettier