diff --whitespace=warn/error: obey blank-at-eof

diff --whitespace=warn/error: obey blank-at-eof

The "diff --check" code used to conflate trailing-space whitespace error
class with this, but now we have a proper separate error class, we should
check it under blank-at-eof, not trailing-space.

The whitespace error is not about _having_ blank lines at end, but about
adding _new_ blank lines.  To keep the message consistent with what is
given by "git apply", call whitespace_error_string() to generate it,
instead of using a hardcoded custom message.

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