patchsend-pack: use internal argv_array of struct child_process

> Avoid a magic number of NULL placeholder values and a magic index by
> constructing the command line for pack-objects using the embedded
> argv_array of the child_process.  The resulting code is shorter and
> easier to extend.
>
> Signed-off-by: Rene Scharfe <l.s.r@web.de>

> ---
>  send-pack.c | 28 +++++++++-------------------
>  1 file changed, 9 insertions(+), 19 deletions(-)
>
> diff --git a/send-pack.c b/send-pack.c
> index a8cc6b266e..2112d3b27a 100644
> --- a/send-pack.c
> +++ b/send-pack.c
> @@ -58,35 +58,25 @@ static int pack_objects(int fd, struct ref *refs, struct oid_array *extra, struc
>          * the revision parameters to it via its stdin and
>          * let its stdout go back to the other end.
>          */
> -       const char *argv[] = {
> -               "pack-objects",
> -               "--all-progress-implied",
> -               "--revs",
> -               "--stdout",
> -               NULL,
> -               NULL,
> -               NULL,
> -               NULL,
> -               NULL,
> -               NULL,
> -       };
>         struct child_process po = CHILD_PROCESS_INIT;
>         FILE *po_in;
>         int i;
>         int rc;
>
> -       i = 4;
> +       argv_array_push(&po.args, "pack-objects");
> +       argv_array_push(&po.args, "--all-progress-implied");
> +       argv_array_push(&po.args, "--revs");
> +       argv_array_push(&po.args, "--stdout");

>         if (args->use_thin_pack)
> -               argv[i++] = "--thin";
> +               argv_array_push(&po.args, "--thin");
>         if (args->use_ofs_delta)
> -               argv[i++] = "--delta-base-offset";
> +               argv_array_push(&po.args, "--delta-base-offset");
>         if (args->quiet || !args->progress)
> -               argv[i++] = "-q";
> +               argv_array_push(&po.args, "-q");
>         if (args->progress)
> -               argv[i++] = "--progress";
> +               argv_array_push(&po.args, "--progress");
>         if (is_repository_shallow())
> -               argv[i++] = "--shallow";
> -       po.argv = argv;
> +               argv_array_push(&po.args, "--shallow");
>         po.in = -1;
>         po.out = args->stateless_rpc ? -1 : fd;
>         po.git_cmd = 1;
> --
> 2.15.1

>> +       argv_array_push(&po.args, "pack-objects");
>> +       argv_array_push(&po.args, "--all-progress-implied");
>> +       argv_array_push(&po.args, "--revs");
>> +       argv_array_push(&po.args, "--stdout");
>
> (useless nit of the day, no need to resend:)
> These four statements could be done as pushl(a,b,c, NULL);
> but it would not differ in readability, so I guess either is fine.