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

It only supports --stdio output currently.  Later patches will extend
additional features.

 $ perf annotate --group --stdio
 ...
  Percent                 |      Source code & Disassembly of libpthread-2.15.so
 --------------------------------------------------------------------------------
                          :
                          :
                          :
                          :      Disassembly of section .text:
                          :
                          :      000000387dc0aa50 <__pthread_mutex_unlock_usercnt>:
     8.08    2.40    5.29 :        387dc0aa50:   mov    %rdi,%rdx
     0.00    0.00    0.00 :        387dc0aa53:   mov    0x10(%rdi),%edi
     0.00    0.00    0.00 :        387dc0aa56:   mov    %edi,%eax
     0.00    0.80    0.00 :        387dc0aa58:   and    $0x7f,%eax
     3.03    2.40    3.53 :        387dc0aa5b:   test   $0x7c,%dil
     0.00    0.00    0.00 :        387dc0aa5f:   jne    387dc0aaa9 <__pthread_mutex_unlock_use
     0.00    0.00    0.00 :        387dc0aa61:   test   %eax,%eax
     0.00    0.00    0.00 :        387dc0aa63:   jne    387dc0aa85 <__pthread_mutex_unlock_use
     0.00    0.00    0.00 :        387dc0aa65:   and    $0x80,%edi
     0.00    0.00    0.00 :        387dc0aa6b:   test   %esi,%esi
     3.03    5.60    7.06 :        387dc0aa6d:   movl   $0x0,0x8(%rdx)
     0.00    0.00    0.59 :        387dc0aa74:   je     387dc0aa7a <__pthread_mutex_unlock_use
     0.00    0.00    0.00 :        387dc0aa76:   subl   $0x1,0xc(%rdx)
     2.02    5.60    1.18 :        387dc0aa7a:   mov    %edi,%esi
     0.00    0.00    0.00 :        387dc0aa7c:   lock decl (%rdx)
    83.84   83.20   82.35 :        387dc0aa7f:   jne    387dc0aada <_L_unlock_586>
     0.00    0.00    0.00 :        387dc0aa81:   nop
     0.00    0.00    0.00 :        387dc0aa82:   xor    %eax,%eax
     0.00    0.00    0.00 :        387dc0aa84:   retq
 ...
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1362462812-30885-6-git-send-email-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>