git删除未跟踪文件
git clean -nf
git clean -nfd
git clean -nxfd
参考:https://blog.csdn.net/uhippo/article/details/46365737
参考:http://wiki.jikexueyuan.com/project/android-source/using-repo.html
http://marklodato.github.io/visual-git-guide/index-zh-cn.html
https://blog.csdn.net/wh_19910525/article/details/8164107