http: use internal argv_array of struct child_process

http: use internal argv_array of struct child_process

Avoid a strangely magic array size (it's slightly too big) and explicit
index numbers by building the command line for index-pack using the
embedded argv_array of the child_process.  Add the flag -o and its
argument with argv_array_pushl() to make it obvious that they belong
together.  The resulting code is shorter and easier to extend.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>