Add option to auto-path to git-root

I like Command-T a lot, but am not quite ready to give up flying around subdirectories while I work in order to use it.  This patch allows the user to set the default search directory to be the first parent directory that contains the ".git" folder.  If it's not found in the upper path, we default back to pwd.

Could be generalized for other SCM marker files and such (I'd be willing to do so if you wanted the patch), but you get the general idea.