备注
一： Staging Modified Files
Let’s change a file that was already tracked. （tracked 表示该文件已经被git管理过，再次修改了文件的状态。对应状态是untracked或not staged）
tracked n:轨道，足迹，踪迹，小道。verb:追踪；通过；循路而行；
二：git add （后该文件为已标记(tracked),状态为Modified ）
git add is a multipurpose command — you use it to begin tracking new files, to stage files, and to do other things like marking merge-conflicted files as resolved. It may be helpful to think of it more as “add precisely this content to the next commit” rather than “add this file to the project”.
purpose1> 跟踪一个新文件
purpose2> 跟踪一个修改后的文件
purpose3> 把合并(特指冲突)后的文件标记为已解决(有点像功能2)
参考资料：https://git-scm.com/book/en/v2/Git-Basics-Recording-Changes-to-the-Repository