Add upstream state to PS1

I'm not sure if this is desired but I like to have the upstream state in my prompt (as supported by [git-prompt](https://github.com/git/git/blob/master/contrib/completion/git-prompt.sh#L51)), e.g.:

```bash
master u=    # same as upstream
master u+3   # ahead of upstream by 3 commits
master u-3   # behind upstream by 3 commits
master u+1-2 # diverged from upstream - I have one commit and they have 2 commits
```

There is also a [less verbose version](https://github.com/git/git/blob/master/contrib/completion/git-prompt.sh#L195) but I didn't add that as I don't use it.