advice: omit trailing whitespace

advice: omit trailing whitespace

Git tools all consistently encourage users to avoid whitespaces at
the end of line by giving them features like "git diff --check" and
"git am --whitespace=fix".  Make sure that the advice messages we
give users avoid trailing whitespaces.  We shouldn't be wasting
vertical screen real estate by adding blank lines in advice messages
that are supposed to be concise hints, but as long as we write such
blank line in our "hints", we should do it right.

A test that expects the current behaviour of leaving trailing
whitespaces has been adjusted.

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