Added ability to specify path of .git folder

I haven't made an ember-cli extensions before so hopefully I didn't violate any conventions. These changes fix the issue for me. I can have my ember-project in a subfolder, specify the `.git` path and when I push the index it has the correct commit hash associated with it.
In `lib/commands/deploy-index.js` I have `git-root` default to an empty string if the user doesn't define one. This will end up using the root. However, I think your change will make it more robust so I'll update my PR.