After setting up Bash Completions via Homebrew, you’ll find that Homebrew will only install Git Completions as part of the entire Git formula.

If you just want to use Completions with Xcode’s built-in Git, you can simply link in Xcode’s copy:

cd /usr/local/etc/bash_completion.d
ln -s /Applications/Xcode.app/Contents/Developer/usr/share/git-core/git-completion.bash