receive-pack: convert push-to-checkout hook to hook.h

receive-pack: convert push-to-checkout hook to hook.h

Move the push-to-checkout hook away from run-command.h to and over to
the new hook.h library.

This removes the last direct user of run_hook_le(), so we could remove
that function now, but let's leave that to a follow-up cleanup commit.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Acked-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>