Homebrew's Git Formula does not appear to include __git_ps1. It is trivial to emulate with a shell alias though, so I did.
Add these lines to ~/.bashrc:
alias __git_ps1="git branch 2>/dev/null | grep '*' | sed 's/* \(.*\)/(\1)/'" case "$TERM" in xterm*|rxvt*) PS1="\[\e]0;\w \$(__git_ps1)\a\]\$> " ;; *) ;; esac