理解工做区/暂存区/版本区(master分支)

git add 是将工做区内容添加到暂存区，git commit是将暂存区内容添加到版本区(master分支)；

$ git diff -- readme.txt                                 //查看工做区与暂存区的差异  
$ git diff HEAD -- readme.txt                            //查看工做区与master分支的差异  
$ git diff --cached -- readme.txt                        //查看暂存区与master分支的差异