-bash: __git_ps1: command not found
となるエラーが出た場合、以下を実行してください。
source /Applications/Xcode.app/Contents/Developer/usr/share/git-core/git-prompt.sh
source /Applications/Xcode.app/Contents/Developer/usr/share/git-core/git-completion.bash
参考 http://sue445.hatenablog.com/entry/2012/08/30/005627
http://biboroku.megaya.net/entry/2015/02/13/002915