perf tools: Add hist.percentage config option

Add hist.percentage option for setting default value of the
symbol_conf.filter_relative.  It affects the output of various perf
commands (like perf report, top and diff) only if filter(s) applied.

An user can write .perfconfig file like below to show absolute
percentage of filtered entries by default:

  $ cat ~/.perfconfig
  [hist]
  percentage = absolute

And it can be changed through command line:

  $ perf report --percentage relative

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/1397145720-8063-6-git-send-email-namhyung@kernel.org
Signed-off-by: Jiri Olsa <jolsa@redhat.com>

tools/perf/builtin-diff.c

diff --git a/tools/perf/builtin-diff.c b/tools/perf/builtin-diff.c
index c903fe1..6ef80f2 100644
--- a/tools/perf/builtin-diff.c
+++ b/tools/perf/builtin-diff.c
@@ -1134,6 +1134,8 @@
 
 int cmd_diff(int argc, const char **argv, const char *prefix __maybe_unused)
 {
+	perf_config(perf_default_config, NULL);
+
 	sort_order = diff__default_sort_order;
 	argc = parse_options(argc, argv, options, diff_usage, 0);