sh-setup: work around "unset IFS" bug in some shells

sh-setup: work around "unset IFS" bug in some shells

With an unset IFS, field splitting is supposed to act as if IFS is
set to the usual SP HT LF, but Marc Branchaud reports that the shell
on FreeBSD 7.2 gets this wrong.

It is easy to set it to the default value manually, and it is also
safer in case somebody tries to save the old value away and restore,
e.g.

	$oIFS=$IFS
	IFS=something
	...
	IFS=$oIFS

while forgetting that the original IFS might be unset (which can be
coded but would be more involved).

Signed-off-by: Junio C Hamano <gitster@pobox.com>