perf sched timehist: Add --next option

perf sched timehist: Add --next option

The --next option shows the next task for each context switch, providing
more context for the sequence of scheduler events.

  $ perf sched timehist --next | head
  Samples do not have callchains.
       time  cpu task name  waittime schdelay run time
                 [tid/pid]     (msec) (msec) (msec)
  ---------- --- ---------- --------- ------ -----
  374.793792 [0] <idle>         0.000  0.000 0.000 next: rngd[1524]
  374.793801 [0] rngd[1524]     0.000  0.000 0.009 next: swapper/0[0]
  374.794048 [7] <idle>         0.000  0.000 0.000 next: yes[30884]
  374.794066 [7] yes[30884]     0.000  0.000 0.018 next: swapper/7[0]
  374.794126 [2] <idle>         0.000  0.000 0.000 next: rngd[1524]
  374.794140 [2] rngd[1524]     0.325  0.006 0.013 next: swapper/2[0]
  374.794281 [3] <idle>         0.000  0.000 0.000 next: perf[31070]

Signed-off-by: Brendan Gregg <bgregg@netflix.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1489456589-32555-1-git-send-email-bgregg@netflix.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>