What's the difference between `fig up -d` and `fig restart`?

`fig up` should build but I noticed it does some kind of build only in some cases, not sure when. So in practice as said for #734 on has to:

```
$ fig build
$ fig up -d
```

but why not just run then `fig restart` (which should do a `fig start` and stop if there is any existing). `fig start` is actually lot faster than `fig up -d`.
