refs: add rejection detail to the callback function

refs: add rejection detail to the callback function

The previous commit started storing the rejection details alongside the
error code for rejected updates. Pass this along to the callback
function `ref_transaction_for_each_rejected_update()`. Currently the
field is unused, but will be integrated in the upcoming commits.

Co-authored-by: Jeff King <peff@peff.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>