completion: zsh: simplify direct compadd

Instead of manually removing the suffix so zsh can add its own, we can
tell zsh to add no suffix, so we don't have to remove it.

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

contrib/completion/git-completion.zsh

@@ -101,7 +101,7 @@ __gitcomp_direct ()
 	emulate -L zsh
 
 	compset -P '*[=:]'
-	compadd -Q -- ${${(f)1}% } && _ret=0
+	compadd -Q -S '' -- ${(f)1} && _ret=0
 }
 
 __gitcomp_direct_append ()