completion: zsh: don't override suffix on _detault

zsh is smart enough to add the right suffix while completing, there's no
point in trying to do the same as bash.

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

contrib/completion/git-completion.zsh

@@ -198,7 +198,7 @@ _git ()
 		emulate ksh -c __${service}_main
 	fi
 
-	let _ret && _default -S '' && _ret=0
+	let _ret && _default && _ret=0
 	return _ret
 }