t6300: check for cat-file exit status code

In test_atom(), we're piping the output of cat-file to tail(1),
thus, losing its exit status.

Let's use a temporary file to preserve git exit status code.

Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6300-for-each-ref.sh

@@ -66,7 +66,9 @@ test_atom() {
 			case $type in
 			tag)
 				# We cannot use $3 as it expects sanitize_pgp to run
-				expect=$(git cat-file tag $ref | tail -n +6 | wc -c) ;;
+				git cat-file tag $ref >out &&
+				expect=$(tail -n +6 out | wc -c) &&
+				rm -f out ;;
 			tree | blob)
 				expect="" ;;
 			commit)