Git has some really nice autocomplete features, but they're not installed by default in OSX. For shame!
Fix this by grabbing this shell script:
curl https://raw.github.com/git/git/master/contrib/completion/git-completion.bash -o ~/.git-completion.bash
Then load it in your bash profile:
if [ -f ~/.git-completion.bash ]; then . ~/.git-completion.bash fi
Bam! Now you have auto-complete for git commands and branch names.
Sourced from this handy blog post: http://code-worrier.com/blog/autocomplete-git/