See which branch you are on immediately after entering a directory with a Git repository. Add this piece of code to your ~/.bash_profile file to run 'git branch' every time you enter a directory with a Git repository. You can comment out line 5 if you do not want to show the Magento version that is being used, if found.

.bash_profile

function runOnFolderEntry() {
  if [ "$PWD" != "$MYOLDPWD" ]; then
    MYOLDPWD="$PWD";
    showGitBranch ${MYOLDPWD};
    showMagentoVersion ${MYOLDPWD};
  fi
}

function showGitBranch() {
    if [ -d "$1/.git" ]; then
        git branch
    fi
}

function showMagentoVersion() {
    if [ -f "$1/app/Mage.php" ]; then
        php -r "include '$1/app/Mage.php'; echo 'Magento version ' . Mage::getVersion(); echo \"\n\";"
    fi
}

export PROMPT_COMMAND=runOnFolderEntry