diff -b: ignore whitespace at end of line

This is _not_ the same as "treat eol as whitespace", since that would mean
that multiple empty lines would be treated as equal to e.g. a space.

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

t/t4015-diff-whitespace.sh

@@ -109,12 +109,10 @@ index d99af23..8b32fb5 100644
 +	whitespace at beginning
  whitespace change
 -whitespace in the middle
--whitespace at end
 +white space in the middle
-+whitespace at end  
+ whitespace at end
  unchanged line
--CR at endQ
-+CR at end
+ CR at endQ
 EOF
 git-diff -b > out
 test_expect_success 'another test, with -b' 'diff -u expect out'