hook: add per-event jobs config

hook: add per-event jobs config

Add a hook.<event>.jobs count config that allows users to override the
global hook.jobs setting for specific hook events.

This allows finer-grained control over parallelism on a per-event basis.

For example, to run `post-receive` hooks with up to 4 parallel jobs
while keeping other events at their global default:

[hook]
    post-receive.jobs = 4

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