t1800: test SIGPIPE with parallel hooks

t1800: test SIGPIPE with parallel hooks

We recently fixed a bug in commit 2226ffaacd (run_processes_parallel():
fix order of sigpipe handling, 2026-04-08) where a hook that caused us
to get SIGPIPE would accidentally trigger the run_processes_parallel()
cleanup handler killing the child processes.

For a single hook, this meant killing the already-exited hook. This case
was triggered by our tests, but was only a problem on some platforms.

But if you have multiple hooks running in parallel, this causes a
problem everywhere, since one hook failing to read its input would take
down all hooks. Now that we have parallel hook support, we can add a
test for this case. It should pass already, due to the existing fix.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>