diff -B: colour whitespace errors

diff -B: colour whitespace errors

We used to send the old and new contents more or less straight out to the
output with only the original "old is red, new is green" colouring.  Now
all the necessary support routines have been prepared, call them with a
line of data at a time from the output code and have them check and color
whitespace errors in exactly the same way as they are called from the low
level diff callback routines.

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