hook: parse the hook.jobs config

hook: parse the hook.jobs config

The hook.jobs config is a global way to set hook parallelization for
all hooks, in the sense that it is not per-event nor per-hook.

Finer-grained configs will be added in later commits which can override
it, for e.g. via a per-event type job options. Next commits will also
add to this item's documentation.

Parse hook.jobs config key in hook_config_lookup_all() and store its
value in hook_all_config_cb.jobs, then transfer it into r->jobs after
the config pass completes.

This is mostly plumbing and the cached value is not yet used.

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