Keep dist/.git while removing everything else.

That way it is possible to have `dist` folder under revision control
for things like hosting on Heroku and push-to-deploy in general.

Thanks @gauntface for a hint on `del` package.
Yup, you need to enable the `dot` option:

```js
del.bind(null, ['.tmp', 'dist/**', '!dist/.git'], {dot: true})
```