理解工做區/暫存區/版本區(master分支)

git add 是將工做區內容添加到暫存區，git commit是將暫存區內容添加到版本區(master分支)；

$ git diff -- readme.txt                                 //查看工做區與暫存區的差異  
$ git diff HEAD -- readme.txt                            //查看工做區與master分支的差異  
$ git diff --cached -- readme.txt                        //查看暫存區與master分支的差異