receive-pack: simplify keep_arg computation

receive-pack: simplify keep_arg computation

To generate "--keep=receive-pack $pid on $host", we write
progressively into a single buffer, which requires keeping
track of how much we've written so far. But since the result
is destined to go into our argv array, we can simply use
argv_array_pushf.

Unfortunately we still have to have a fixed-size buffer for
the gethostname() call, but at least it now doesn't involve
any extra size computation. And as a bonus, we drop an
sprintf and a strcpy call.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>