Add missing -z to git check-attr usage text for consistency with man page

Signed-off-by: Adam Spiers <git@adamspiers.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/check-attr.c

diff --git a/builtin/check-attr.c b/builtin/check-attr.c
index 44c421e..9000c2d 100644
--- a/builtin/check-attr.c
+++ b/builtin/check-attr.c
@@ -9,7 +9,7 @@
 static int stdin_paths;
 static const char * const check_attr_usage[] = {
 "git check-attr [-a | --all | attr...] [--] pathname...",
-"git check-attr --stdin [-a | --all | attr...] < <list-of-paths>",
+"git check-attr --stdin [-z] [-a | --all | attr...] < <list-of-paths>",
 NULL
 };