Updated to use porcelain Git commands

Updated find_git_branch to use git rev-parse, and updated find_git_dirty to use git diff-files. I think this solves the issue with submodules, and makes it a little more robust (i.e., for feature / release branches from git-flow).

I also changed the function style to reflect what is used in some of git-core, and I removed the preceding space in the branch name (like another Git prompt tutorial had -- forgot the link) for personal preference.

Thank you for this awesome utility!