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

contrib/completion/git-completion.bash

@@ -2404,6 +2404,8 @@ __gitk_main ()
 if [[ -n ${ZSH_VERSION-} ]]; then
 	echo "WARNING: this script is deprecated, please see git-completion.zsh" 1>&2
 
+	autoload -U +X compinit && compinit
+
 	__gitcomp ()
 	{
 		emulate -L zsh