hook API: fix v2.36.0 regression: hooks should be connected to a TTY

Fix a regression reported[1] against f443246b9f (commit: convert
{pre-commit,prepare-commit-msg} hook to hook.h, 2021-12-22): Due to
using the run_process_parallel() API in the earlier 96e7225b31 (hook:
add 'run' subcommand, 2021-12-22) we'd capture the hook's stderr and
stdout, and thus lose the connection to the TTY in the case of
e.g. the "pre-commit" hook.

As a preceding commit notes GNU parallel's similar --ungroup option
also has it emit output faster. While we're unlikely to have hooks
that emit truly massive amounts of output (or where the performance
thereof matters) it's still informative to measure the overhead. In a
similar "seq" test we're now ~30% faster:

	$ cat .git/hooks/seq-hook; git hyperfine -L rev origin/master,HEAD~0 -s 'make CFLAGS=-O3' './git hook run seq-hook'
	#!/bin/sh

	seq 100000000
	Benchmark 1: ./git hook run seq-hook' in 'origin/master
	  Time (mean ± σ):     787.1 ms ±  13.6 ms    [User: 701.6 ms, System: 534.4 ms]
	  Range (min … max):   773.2 ms … 806.3 ms    10 runs

	Benchmark 2: ./git hook run seq-hook' in 'HEAD~0
	  Time (mean ± σ):     603.4 ms ±   1.6 ms    [User: 573.1 ms, System: 30.3 ms]
	  Range (min … max):   601.0 ms … 606.2 ms    10 runs

	Summary
	  './git hook run seq-hook' in 'HEAD~0' ran
	    1.30 ± 0.02 times faster than './git hook run seq-hook' in 'origin/master'

1. https://lore.kernel.org/git/CA+dzEBn108QoMA28f0nC8K21XT+Afua0V2Qv8XkR8rAeqUCCZw@mail.gmail.com/

Reported-by: Anthony Sottile <asottile@umich.edu>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
[jc: minor fix-up to tests for consistency]
Signed-off-by: Junio C Hamano <gitster@pobox.com>

hook.c

@@ -144,6 +144,7 @@ int run_hooks_opt(const char *hook_name, struct run_hooks_opt *options)
 		cb_data.hook_path = abs_path.buf;
 	}
 
+	run_processes_parallel_ungroup = 1;
 	run_processes_parallel_tr2(jobs,
 				   pick_next_hook,
 				   notify_start_failure,