$ max @ Maxs-MBP-2016 in ~/Develop/learnhtml on git:master o
$ git checkout 5009f3a5f4
Note: checking out '5009f3a5f4'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by performing another checkout.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -b with the checkout command again. Example:

  git checkout -b <new-branch-name>
大专栏  Learn Git Lesson06 - 分离头指针
HEAD is now at 5009f3a Add index.html

$ max @ Maxs-MBP-2016 in ~/Develop/learnhtml on git:5009f3a o 
$ ls
README.md  index.html

$ max @ Maxs-MBP-2016 in ~/Develop/learnhtml on git:5009f3a o 
$ vim README.md

$ max @ Maxs-MBP-2016 in ~/Develop/learnhtml on git:5009f3a x
$ git checkout master

$ max @ Maxs-MBP-2016 in ~/Develop/learnhtml on git:master x 
$ Git diff a8a599a4dd07 0434ef25597
$ Git diff HEAD HEAD~1
$ Git diff HEAD HEAD~2