perf diff: color the Weighted Diff column

In

  $ perf diff -c wdiff:M,N

color the numbers in the Weighted Diff column using color_snprintf(),
picking the colors using get_percent_color().

Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>