检查状态，看看发生了什么：

$ git status

On branch master

Changed but not updated:

(use "git add/rm <file>..." to update what will be committed)

(use "git checkout -- <file>..." to discard changes in working directory)

deleted: app/controllers/application_controller.rb

no changes added to commit (use "git add" and/or "git commit -a")

可以看出，我们删除了一个文件。但是这个改动只发生在工作区（working tree）中，还未提交到仓库。这意

味着，我们可以使用checkout 命令，并指定-f 旗标，强制撤销这次改动：

$ git checkout -f

$ git status

# On branch master

nothing to commit (working directory clean)

$ ls app/controllers/

application_controller.rb concerns/

删除的目录和文件又回来了，ohlala！