Cast execl*() NULL sentinels to (char *)

Cast execl*() NULL sentinels to (char *)

The NULL sentinel argument to the execl*() family of calls must be
cast to (char *), as otherwise:

- platforms where NULL is just 0 (not (void *)) would pass an int

- (admittedly esoteric) platforms where NULL is (void *)0 and (void *)
  and (char *) have different memory layouts would pass the wrong kind
  of pointer

Signed-off-by: Thomas Rast <redacted>
Signed-off-by: Junio C Hamano <redacted>