The POSIX-function fork is not supported on Windows. Use our
start_command API instead, respawning ourselves in a special
"writer" mode to follow the alternate code path.
Remove the NOT_MINGW-prereq for t5000, as git-archive --remote
now works.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>