perf hists: Add missing period_* fields when collapsing a hist entry

perf hists: Add missing period_* fields when collapsing a hist entry

So that the perf report won't lost the cpu utilization information.

For example, if there're two process that have same name.

  $ perf report --stdio --showcpuutilization -s pid
  [SNIP]
  #   Overhead       sys        us  Command:  Pid
  #   ........  ........  ........  .............
  #
        55.12%     0.01%    55.10%  noploop:28781
        44.88%     0.06%    44.83%  noploop:28782

Before:
  $ perf report --stdio --showcpuutilization -s comm
  [SNIP]
  #   Overhead       sys        us
  #   ........  ........  ........
  #
       100.00%     0.06%    44.83%

After:
  $ perf report --stdio --showcpuutilization -s comm
  [SNIP]
  #   Overhead       sys        us
  #   ........  ........  ........
  #
       100.00%     0.07%    99.93%

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Arun Sharma <asharma@fb.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.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/1348645663-25303-2-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>