$ cd ~
$ wget https://raw.githubusercontent.com/bobthecow/git-flow-completion/master/git-flow-completion.zsh
$ mv git-flow-completion.zsh .git-flow-completion.zsh

Agregar en el archivo .zshrc la línea

source ~/.git-flow-completion.zsh

Fuente:

https://github.com/bobthecow/git-flow-completion