hook: allow disabling config hooks

hook: allow disabling config hooks

Hooks specified via configs are always enabled, however users
might want to disable them without removing from the config,
like locally disabling a global hook.

Add a hook.<name>.enabled config which defaults to true and
can be optionally set for each configured hook.

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>