cat-file --batch: flush stdout also when objects are missing

cat-file --batch/--batch-check only flushes stdout when the object
exists, but not when it doesn't ("<object> missing").  This makes
bidirectional pipes hang.

Signed-off-by: Lea Wiemann <LeWiemann@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-cat-file.c

@@ -159,6 +159,7 @@ static int batch_one_object(const char *obj_name, int print_contents)
 
 	if (get_sha1(obj_name, sha1)) {
 		printf("%s missing\n", obj_name);
+		fflush(stdout);
 		return 0;
 	}