perf report: Show call graph from reference events

Introduce --show-ref-call-graph for perf report to print reference
callgraph for no callgraph event.

Here is an example.

 perf report --show-ref-call-graph --stdio

 # To display the perf.data header info, please use
 --header/--header-only options.
 #
 #
 # Total Lost Samples: 0
 #
 # Samples: 5  of event 'cpu/cpu-cycles,call-graph=fp/'
 # Event count (approx.): 144985
 #
 # Children      Self  Command  Shared Object     Symbol
 # ........  ........  .......  ................  ........................................
 #
    72.30%     0.00%  sleep    [kernel.vmlinux]  [k] entry_SYSCALL_64_fastpath
              |
              ---entry_SYSCALL_64_fastpath
                 |
                 |--22.62%-- __GI___libc_nanosleep
                  --77.38%-- [...]

......

 # Samples: 6  of event 'cpu/instructions,call-graph=no/', show reference callgraph
 # Event count (approx.): 172780
 #
 # Children      Self  Command  Shared Object     Symbol
 # ........  ........  .......  ................  ........................................
 #
    73.16%     0.00%  sleep    [kernel.vmlinux]  [k] entry_SYSCALL_64_fastpath
              |
              ---entry_SYSCALL_64_fastpath
                 |
                 |--31.44%-- __GI___libc_nanosleep
                  --68.56%-- [...]

Signed-off-by: Kan Liang <kan.liang@intel.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/1439289050-40510-3-git-send-email-kan.liang@intel.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>