perf diff: Color the Ratio column

In

  $ perf diff -c ratio

color the Ratio column using value_color_snprintf(), a new function that
operates exactly like percent_color_snprintf().

At first glance, it looks like percent_color_snprintf() can be turned
into a non-variadic function simplifying things; however, 53805eca

 (perf
tools: Remove cast of non-variadic function to variadic, 2013-10-31)
explains why it needs to be a variadic function.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/1388388861-7931-4-git-send-email-artagnon@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>