cat-file: use oidset check-and-insert

We don't need to check if the oidset has our object before
we insert it; that's done as part of the insertion. We can
just rely on the return value from oidset_insert(), which
saves one hash lookup per object.

This measurable speedup is tiny and within the run-to-run
noise, but the result is simpler to read, too.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/cat-file.c

@@ -443,9 +443,8 @@ static int batch_unordered_object(const struct object_id *oid, void *vdata)
 {
 	struct object_cb_data *data = vdata;
 
-	if (oidset_contains(data->seen, oid))
+	if (oidset_insert(data->seen, oid))
 		return 0;
-	oidset_insert(data->seen, oid);
 
 	return batch_object_cb(oid, data);
 }