diff --color: color blank-at-eof

diff --color: color blank-at-eof

Since the coloring logic processed the patch output one line at a time, we
couldn't easily color code the new blank lines at the end of file.

Reuse the adds_blank_at_eof() function to find where the runs of such
blank lines start, keep track of the line number in the preimage while
processing the patch output one line at a time, and paint the new blank
lines that appear after that line to implement this.

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