completion: zsh: trivial simplification

>From upstream bash simplification:

  d9ee1e0617 (completion: simplify inner 'case' pattern in __gitcomp())

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

contrib/completion/git-completion.zsh

@@ -85,7 +85,7 @@ __gitcomp ()
 			fi
 			c="$c${4-}"
 			case $c in
-			--*=*|*.) ;;
+			--*=|*.) ;;
 			*) c="$c " ;;
 			esac
 			array+=("$c")