perf diff: Use internal rb tree for compute resort

perf diff: Use internal rb tree for compute resort

There's no reason to run hists_compute_resort() using output tree.
Convert it to use internal tree so that it can remove unnecessary
_output_resort.

Signed-off-by: Namhyung Kim <[email protected]>
Acked-by: Jiri Olsa <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephane Eranian <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>