git-sh-setup sets IFS but it is not used by git-difftool--helper.
Set IFS in git-mergetool--lib so that the mergetool scriptlets,
difftool, and mergetool do not need to do so.
Signed-off-by: David Aguilar <davvid@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>