Run this command to get the .git-completion.bash file downloaded to your home directory.
curl https://raw.github.com/git/git/master/contrib/completion/git-completion.bash -o ~/.git-completion.bash
Add the following code to your .bash_profile which is located in the root of your home directory.
if [ -f ~/.git-completion.bash ]; then . ~/.git-completion.bash fi
Now when you open up your shell you will be able to use the TAB key for your Git commands.