hook: add "git hook list" command

hook: add "git hook list" command

The previous commit introduced an ability to run multiple commands for
hook events and next commit will introduce the ability to define hooks
from configs, in addition to the "traditional" hooks from the hookdir.

Introduce a new command "git hook list" to make inspecting hooks easier
both for users and for the tests we will add.

Further commits will expand on this, e.g. by adding a -z output mode.

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