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/util/hist.h

@@ -258,5 +258,6 @@ unsigned int hists__sort_list_width(struct hists *hists);
 struct option;
 int parse_filter_percentage(const struct option *opt __maybe_unused,
 			    const char *arg, int unset __maybe_unused);
+int perf_hist_config(const char *var, const char *value);
 
 #endif	/* __PERF_HIST_H */