We do not yet need both a rollback and a free function for transactions.
Remove ref_transaction_rollback and use ref_transaction_free instead.
At a later stage we may reintroduce a rollback function if we want to start
adding reusable transactions and similar.
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Ronnie Sahlberg <sahlberg@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com> Acked-by: Michael Haggerty <mhagger@alum.mit.edu>