One question on git diff command
 

I am a newbie of git, and I found the output is not my expected.

I have two branches , master and testing. In testing, I added one file such as file2.c.

Then I switch back to master branch, and try to merge this to master.
I can see this file2.c is in the master branch now. And git status shows, this file is ready to commit.

But when I use git diff, there shows no diff.

So how can I show this kind of diff?

BTW, how about to revert this merge, if I think this is not a good time to do this?