use child_process_init() to initialize struct child_process variables

use child_process_init() to initialize struct child_process variables

Call child_process_init() instead of zeroing the memory of variables of
type struct child_process by hand before use because the former is both
clearer and shorter.

Signed-off-by: Rene Scharfe <redacted>
Signed-off-by: Junio C Hamano <redacted>