hook: add -j/--jobs option to git hook run

hook: add -j/--jobs option to git hook run

Expose the parallel job count as a command-line flag so callers can
request parallelism without relying only on the hook.jobs config.

Add tests covering serial/parallel execution and TTY behaviour under
-j1 vs -jN.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Helped-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>