hook: convert 'post-rewrite' hook in sequencer.c to hook API

hook: convert 'post-rewrite' hook in sequencer.c to hook API

Replace the custom run-command calls used by post-rewrite with
the newer and simpler hook_run_opt(), which does not need to
create a custom 'struct child_process' or call find_hook().

Another benefit of using the hook API is that hook_run_opt()
handles the SIGPIPE toggle logic.

Signed-off-by: Emily Shaffer <redacted>
Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Adrian Ratiu <redacted>
Signed-off-by: Junio C Hamano <redacted>