After procrastinating for too long, I finally spent a minute of my life to setup git autocomplete in the macOS terminal. The original discussion on this topic is found here.

First, run the following command in Terminal to download the autocomplete script:

curl https://raw.githubusercontent.com/git/git/master/contrib/completion/git-completion.bash -o ~/.git-completion.bash

This will download the autocomplete script and place it as a hidden file in your home folder. After this, open (or create if you have none) ~/.bash_profile and add the following to it:

test -f ~/.git-completion.bash && . $_

This will make the Terminal run the .git-completion.bash script if it exists.

After this, restart Terminal or open a new tab to apply this change. You should now be able to autocomplete branch names, git commands etc.