upload-archive: use start_command instead of fork

upload-archive: use start_command instead of fork

The POSIX-function fork is not supported on Windows. Use our
start_command API instead.

As this is the last call-site that depends on the fork-stub in
compat/mingw.h, remove that as well.

Add an undocumented flag to git-archive that tells it that the
action originated from a remote, so features can be disabled.
Thanks to Jeff King for work on this part.

Remove the NOT_MINGW-prereq for t5000, as git-archive --remote
now works.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>