receive-pack: print --pack-header directly into argv array

receive-pack: print --pack-header directly into argv array

After receive-pack reads the pack header from the client, it
feeds the already-read part to index-pack and unpack-objects
via their --pack-header command-line options.  To do so, we
format it into a fixed buffer, then duplicate it into the
child's argv_array.

Our buffer is long enough to handle any possible input, so
this isn't wrong. But it's more complicated than it needs to
be; we can just argv_array_pushf() the final value and avoid
the intermediate copy. This drops the magic number and is
more efficient, too.

Note that we need to push to the argv_array in order, which
means we can't do the push until we are in the "unpack-objects
versus index-pack" conditional.  Rather than duplicate the
slightly complicated format specifier, I pushed it into a
helper function.

Signed-off-by: Jeff King <peff@peff.net>