If you faced with problem that out-of-the-box Git under Mac has no auto-complete functionality you can go this way:

curl https://github.com/git/git/raw/master/contrib/completion/git-completion.bash -OL

Copy it to, for example,

cp git-completion.bash ~/.git-completion.bash

Then open your ~/.bash_profile or ~/.profile and append next string

source ~/.git-completion.bash

Thats’s it 🙂