There are two ways to set up git auto-completion on macOS.

Option 1: drop in the completion script directly

curl https://raw.githubusercontent.com/git/git/master/contrib/completion/git-completion.bash   -o ~/.git-completion.bash   && echo "source ~/.git-completion.bash" >> ~/.bash_profile   && source ~/.bash_profile

Paste it into your terminal and that’s it.

Option 2: install via Homebrew

If you don’t already have git installed:

brew install git && brew install bash-completion

If git is already installed:

brew install bash-completion

Then add bash-completion to your ~/.bash_profile:

if [ -f $(brew --prefix)/etc/bash_completion ]; then
  . $(brew --prefix)/etc/bash_completion
fi

Source your profile or open a new terminal and auto-completion will be active.

Hope this helps.