completion: zsh: fix for command aliasing

A lot of people want to define aliases like gc='git commit', and zsh allows that (when not using 'complete_aliases'), but we need to handle services that call a function other than the main one. With this patch we can do: compdef _git gc=git_commit Additionally, add compatibility for Zsh Git functions which have the form git-commit (with dash, not underscore). Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>