update-ref: allow creation of multiple transactions

update-ref: allow creation of multiple transactions

While git-update-ref has recently grown commands which allow interactive
control of transactions in e48cf33b61 (update-ref: implement interactive
transaction handling, 2020-04-02), it is not yet possible to create
multiple transactions in a single session. To do so, one currently still
needs to invoke the executable multiple times.

This commit addresses this shortcoming by allowing the "start" command
to create a new transaction if the current transaction has already been
either committed or aborted.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>