reference-transaction: use hook API instead of run-command

reference-transaction: use hook API instead of run-command

Convert the reference-transaction hook to the new hook API,
so it doesn't need to set up a struct child_process, call
find_hook or toggle the pipe signals.

The stdin feed callback is processing one ref update per
call. I haven't noticed any performance degradation due
to this, however we can batch as many we want in each call,
to ensure a good pipe throughtput (i.e. the child does not
wait after stdin).

Helped-by: Emily Shaffer <nasamuffin@google.com>
Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>