Show how many commits this branch is ahead/behind the remote
Use > and < instead of + and - to show ahead and behind count
Do not mark dirty solely due to untracked files
Use square brackets instead of parentheses
Add colors to ahead/behind counts
Add $ at end of prompt, if branch is showing (assumes git_dirty is the last thing displayed in prompt.)
Add a gap instead of $
Instead of showing commit count, just show a minimalist marker
Separate ahead and behind markers and count, so they can be colored by the prompt string