refs.c: add transaction.status and track OPEN/CLOSED
Track the state of a transaction in a new state field. Check the field for
sanity, i.e. that state must be OPEN when _commit/_create/_delete or
_update is called or else die(BUG:...)
Signed-off-by: Ronnie Sahlberg <sahlberg@google.com> Reviewed-by: Michael Haggerty <mhagger@alum.mit.edu> Signed-off-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>