Fix "git status" parsing in prompt for git 1.8.5

With the release of git 1.8.5 they are removing the `#` symbol from the output of `git status`.  This causes an issue when there are staged files to be committed where `git_dirty()` returns an empty string thus causing the branch information to disappear from the prompt.

Looking through the changelog they recommend using `git status --porcelain` to parse from because the out is more stable.  To that end I've attempted to do that, and wanted to submit it back.