.bash_prompt: Fix `parse_git_dirty` for Git v1.8.0+
.bash_prompt: Fix `parse_git_dirty` for Git v1.8.0+

Git 1.8.0+ says “nothing to commit (working directory clean)”.
Older Git versions return “nothing to commit (working directory clean)”.

Thanks to @nicolahery in #143 for the suggestion.

VPusher/dotfiles,VPusher/dotfiles