perf tools: Generalize percent_color_snprintf()

Make percent_color_snprintf() handle negative values correctly.

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/1388388861-7931-2-git-send-email-artagnon@gmail.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>