completion: fix warning for zsh

Otherwise the user might get something like:

git-completion.sh:2466: command not found: compdef

If this script is loaded before compinit. The script would work either
way, but let's not be more annoying to the user.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>