git help add
-A, --all
Like -u, but match <filepattern> against files in the working tree in addition to the index. That means that it will find new files as well as staging modified content and removing files that are no longer in the working tree.也就是发现的所有新增、删除、修改的文件都不在working tree上， 就会working directory clean了。
参考： http://www.crifan.com/git_add_help_output/