update-ref: fix streaming of status updates
When executing git-update-ref(1) with the `--stdin` flag, then the user
can queue updates and, since
e48cf33b61 (update-ref: implement
interactive transaction handling, 2020-04-02), interactively drive the
transaction's state via a set of transactional verbs. This interactivity
is somewhat broken though: while the caller can use these verbs to drive
the transaction's state, the status messages which confirm that a verb
has been processed is not flushed. The caller may thus be left hanging
waiting for the acknowledgement.
Fix the bug by flushing stdout after writing the status update. Add a
test which catches this bug.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>