"Error" --unshallow on a complete repository does not make sense

<img width="895" alt="Screen Shot 2019-09-05 at 11 46 20 AM" src="https://user-images.githubusercontent.com/2193314/64370213-da0ede00-cfd2-11e9-9598-697aff434e39.png">

I got this error as I was doing a `git pull` in the integrated terminal on https://github.com/xtermjs/xterm.js/pull/2323 checked out branch.