completion: zsh: fix for command aliasing

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>