To make zsh complete these commands:

    zstyle ':completion:*:*:git:*' user-commands ${${(M)${(k)commands}:#git-*}/git-/}

Or specify just the ones you want to be completed:

    zstyle ':completion:*:*:git:*' user-commands foo:'description for foo'

(from /usr/share/zsh/functions/Completion/Unix/_git)