perf report: Add --group option

Add --group option to enable event grouping.  When enabled, all the
group members information will be shown together with the leader.

  $ perf report --group
  ...
  # group: {ref-cycles,cycles}
  # ========
  #
  # Samples: 7K of event 'anon group { ref-cycles, cycles }'
  # Event count (approx.): 6876107743
  #
  #         Overhead  Command      Shared Object                      Symbol
  # ................  .......  .................  ..........................
  #
      99.84%  99.76%  noploop  noploop            [.] main
       0.07%   0.00%  noploop  ld-2.15.so         [.] strcmp
       0.03%   0.00%  noploop  [kernel.kallsyms]  [k] timerqueue_del
       0.03%   0.03%  noploop  [kernel.kallsyms]  [k] sched_clock_cpu
       0.02%   0.00%  noploop  [kernel.kallsyms]  [k] account_user_time
       0.01%   0.00%  noploop  [kernel.kallsyms]  [k] __alloc_pages_nodemask
       0.00%   0.00%  noploop  [kernel.kallsyms]  [k] native_write_msr_safe
       0.00%   0.11%  noploop  [kernel.kallsyms]  [k] _raw_spin_lock
       0.00%   0.06%  noploop  [kernel.kallsyms]  [k] find_get_page
       0.00%   0.02%  noploop  [kernel.kallsyms]  [k] rcu_check_callbacks
       0.00%   0.02%  noploop  [kernel.kallsyms]  [k] __current_kernel_time

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1358845787-1350-18-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-report.txt

@@ -203,6 +203,9 @@ OPTIONS
 --objdump=<path>::
         Path to objdump binary.
 
+--group::
+	Show event group information together.
+
 SEE ALSO
 --------
 linkperf:perf-stat[1], linkperf:perf-annotate[1]

tools/perf/builtin-report.c

@@ -668,6 +668,8 @@ int cmd_report(int argc, const char **argv, const char *prefix __maybe_unused)
 		   "Specify disassembler style (e.g. -M intel for intel syntax)"),
 	OPT_BOOLEAN(0, "show-total-period", &symbol_conf.show_total_period,
 		    "Show a column with the sum of periods"),
+	OPT_BOOLEAN(0, "group", &symbol_conf.event_group,
+		    "Show event group information together"),
 	OPT_CALLBACK_NOOPT('b', "branch-stack", &sort__branch_mode, "",
 		    "use branch records for histogram filling", parse_branch_mode),
 	OPT_STRING(0, "objdump", &objdump_path, "path",