builtin/receive-pack: avoid spinning no-op sideband async threads

builtin/receive-pack: avoid spinning no-op sideband async threads

Exit early if the hooks do not exist, to avoid spinning up/down
sideband async threads which no-op.

It is important to call the hook_exists() API provided by hook.[ch]
because it covers both config-defined hooks and the "traditional"
hooks from the hookdir. find_hook() only covers the hookdir hooks.

The regression happened because the no-op async threads add some
additional overhead which can be measured with the receive-refs test
of the benchmarks suite [1].

Reproduced using:
cd benchmarks/receive-refs && \
./run --revisions /path/to/git \
fc148b146ad41be71a7852c4867f0773cbfe1ff9~,fc148b146ad41be71a7852c4867f0773cbfe1ff9 \
--parameter-list refformat reftable --parameter-list refcount 10000

1: https://gitlab.com/gitlab-org/data-access/git/benchmarks

Fixes: fc148b146ad4 ("receive-pack: convert update hooks to new API")
Reported-by: Patrick Steinhardt <ps@pks.im>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
[jc: avoid duplicated hardcoded hook names]
Signed-off-by: Junio C Hamano <gitster@pobox.com>