まず、「git status」でUntracked filesを確認
zsh
% git status
Untracked files:
(use "git add <file>..." to include in what will be committed)
xxx/xxx
yyy/yyy
「git clean」に「nオプション」を付け、削除するUntracked filesを確認
zsh
% git clean -n
Would remove xxx/xxx
Would remove yyy/yyy
「git clean」に「fオプション」を付け、Untracked filesを削除
zsh
% git clean -f
Removing xxx/xxx
Removing yyy/yyy