add git done

Run this when your Pull Request has been accepted and you're ready to merge
into master.

* Fetches and rebases your branch (interactively) onto origin/master
* Pushes up the new changes into your branch so that the Pull Request will be
 closed automatically.
* Merges the changes into your local master so that you get any upstream
 changes as well as the changes from your branch.
* Pushes to origin
* Deletes your branch on origin and master