t7004: give the test a bit more stack space

It was reported that the allocated stack space was too small for
some archs openSUSE buildfarm runs the tests on.  Double it while
also doubling the amount of data to be handled.

Reported-by: Andreas Schwab <schwab@linux-m68k.org>
Suggested-by: Jeff King <peff@peff.net>
Tested-by: Andreas Schwab <schwab@linux-m68k.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>