How can I configure zsh to autocomplete branch names while using 'git checkout'?
The easiest way (and probably the only one not requiring a serious amount of hacking) is to update zsh. On my zsh 4.3.9 autocompletion works for git branches.
I got git autocomplete to work in zsh by adding this to my .zshrc:
autoload -Uz compinit && compinit