http://www.kafeitu.me/git/2012/03/27/git-submodule.html
https://git-scm.com/book/zh/v2/Git-%E5%B7%A5%E5%85%B7-%E5%AD%90%E6%A8%A1%E5%9D%97
http://www.kafeitu.me/git/2012/03/27/git-submodule.html
https://git-scm.com/book/zh/v2/Git-%E5%B7%A5%E5%85%B7-%E5%AD%90%E6%A8%A1%E5%9D%97