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.
Hi @sindresorhus , I was thinking about the same thing as you but I tested this (with one globstar) and it actually removes all the subfolders (with two globstars it removes the whole dist folder).