completion: fix warning for zsh
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 <de77a8b9cd239b204c19c52ed62e48ab47243362@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git