There are many questions about git, so I google for a while before I asked here. I've not find any useful info.
Just make it easy,
cat a.txt >> c.txt
git add .
cat b.txt >> c.txt
how to undo the last cat command.
Note here is just a simple to illustrate what I want, there are lot of modify before and after the git add.
The question may be duplicate, or the title is not proper, may someone helps?