Today I updated mac ports and a new error started to show up at the command line saying that the __git_ps1 command is not found.
To fix it, you just need to include following lines in your .profile (or .bashrc):
if [ -f /opt/local/share/doc/git-core/contrib/completion/git-prompt.sh ]; then . /opt/local/share/doc/git-core/contrib/completion/git-prompt.sh fi
It worked for me.
Depending on your git installation, this file may be located elsewhere; find command should help in this case.