Level: Staging - `git rm --cached`

This level would tell you how to remove things from the staging already

The level would behave like this:
```
$ git gud goal
Commit the files one at a time
$ git status
Changes to be committed:
commitmefirst.txt // "Haha, my contents have changed!"
commitmesecond.txt // "Gosh, I hope there are no typeos"
$ git rm --cached commitmesecond.txt
$ git commit
$ git gud test
Level completed!
```