fetch-pack: use argv_array for index-pack / unpack-objects

fetch-pack: use argv_array for index-pack / unpack-objects

This cleans up a magic number that must be kept in sync with
the rest of the code (the number of argv slots). It also
lets us drop some fixed buffers and an sprintf (since we
can now use argv_array_pushf).

We do still have to keep one fixed buffer for calling
gethostname, but at least now the size computations for it
are much simpler.

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