diff --check: do not unconditionally complain about trailing empty lines

diff --check: do not unconditionally complain about trailing empty lines

Recently "git diff --check" learned to detect new trailing blank lines
just like "git apply --whitespace" does.  However this check should not
trigger unconditionally.  This patch makes it honor the whitespace
settings from core.whitespace and gitattributes.

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