hooks: convert non-worktree 'post-checkout' hook to hook library

hooks: convert non-worktree 'post-checkout' hook to hook library

Move the running of the 'post-checkout' hook away from run-command.h
to the new hook.h library, except in the case of
builtin/worktree.c. That special-case will be handled in a subsequent
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>