命令：
gir diff gir diff --staged
1. 查看某个文件修改了哪些内容，后者是查看所有文件都修改了哪些内容（注意：这是查看未追踪的文件的）
git diff 文件名git difff
2. 查看（已追踪但未提交）的文件修改差异
git diff --staged
