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！