hook: allow hook.jobs=-1 to use all available CPU cores

hook: allow hook.jobs=-1 to use all available CPU cores

Allow -1 as a value for hook.jobs, hook.<event>.jobs, and the -j
CLI flag to mean "use as many jobs as there are CPU cores", matching
the convention used by fetch.parallel and other Git subsystems.

The value is resolved to online_cpus() at parse time so the rest
of the code always works with a positive resolved count.

Other non-positive values (0, -2, etc) are rejected with a warning
(config) or die (CLI).

Suggested-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>