参考资料：    1. http://www.liaoxuefeng.com/wiki/0013739516305929606dd18361248578c67b8067c8c017b000/    2. http://www.cnblogs.com/cassvin/archive/2012/01/29/git_commands.html    3. https://git-scm.com/book/zh/v2    4. http://gitref.org/zh/index.html