Traverse scm

One of the features i quite liked about ctrlp (though i prefer the command-t ui) is the ability to have it set the root dir for a file find to the nearest parent determined to ge an scm root. there is already a [pull request](https://github.com/wincent/Command-T/pull/29) adding this but it finds the root based on `pwd` whereas i want the root based on the file currently being edited