exclude .git folder when init'ing a local template

Like [a few other people on the middleman forum](http://forum.middlemanapp.com/t/templates-doubts-post-install-note-and-not-keeping-the-git-history/1017/), I noticed the git history is brought along when a new project is `init`'d using a local template whose source is a git repo.

This behavior doesn't seem right, so I fixed it. (See my comment on the aforementioned forum post.)

I hope you agree my patch is the Right Way to do it!

-Justin