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>
HEAD is now at 11dfa53... Added a.txt
------- 就是说tag是一个独立的存在，不能被修改，就算修改了，即使commit了，立马就到一个新的head上了，不再是在这个tag上了。
为什么说tag不能修改呢。