git-send-email: Respect core.hooksPath setting

git-send-email: Respect core.hooksPath setting

get-send-email currently makes the assumption that the
'sendemail-validate' hook exists inside of the repository.

Since the introduction of 'core.hooksPath' configuration option in
867ad08a261 (hooks: allow customizing where the hook directory is,
2016-05-04), this is no longer true.

Instead of assuming a hardcoded repo relative path, query
git for the actual path of the hooks directory.

Signed-off-by: Robert Foss <robert.foss@linaro.org>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>