completion: zsh: add simple version check

A lot of people are confused about which completion script they are
using; Zsh's Git script, or Git's Zsh script.

Add a simple helper so they can type 'git zsh<tab>' and find out if they
are running the correct one: this.

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

contrib/completion/git-completion.zsh

@@ -134,6 +134,11 @@ __gitcomp_file_direct ()
 	__gitcomp_file "$1" ""
 }
 
+_git_zsh ()
+{
+	__gitcomp "v1.1"
+}
+
 __git_complete_command ()
 {
 	emulate -L zsh