completion: regression fix for zsh

zsh completion wrapper doesn't reimplement __gitcompadd(). Although it
should be trivial to do that, let's use __gitcomp_nl() which achieves
exactly the same thing, specially since the suffix ($4) has to be empty.
Signed-off-by: NFelipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1831,7 +1831,7 @@ _git_config ()
 		local remote="${prev#remote.}"
 		remote="${remote%.fetch}"
 		if [ -z "$cur" ]; then
-			__gitcompadd "refs/heads/" "" "" ""
+			__gitcomp_nl "refs/heads/" "" "" ""
 			return
 		fi
 		__gitcomp_nl "$(__git_refs_remotes "$remote")"