hook: add hook.<event>.enabled switch

Add a hook.<event>.enabled config key that disables all hooks for
a given event, when set to false, acting as a high-level switch
above the existing per-hook hook.<friendly-name>.enabled.

Event-disabled hooks are shown in "git hook list" with an
"event-disabled" tab-separated prefix before the name:

$ git hook list test-hook
event-disabled  hook-1
event-disabled  hook-2

With --show-scope:

$ git hook list --show-scope test-hook
local   event-disabled  hook-1

When a hook is both per-hook disabled and event-disabled, only
"event-disabled" is shown: the event-level switch is the more
relevant piece of information, and the per-hook "disabled" status
will surface once the event is re-enabled.

Using an event name as a friendly-name (e.g. hook.<event>.enabled)
can cause ambiguity, so a fatal error is issued when using a known
event name and a warning is issued for unknown event name, since
a collision cannot be detected with certainty for unknown events.

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

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