perf diff: Fix -o/--order option behavior

perf diff: Fix -o/--order option behavior

The prior change fixes default output ordering with each column but it
breaks -o/--order option.  This patch prepends a new hpp fmt struct to
sort list but not to output field list so that it can affect ordering
without adding a new output column.

The new hpp fmt uses its own compare functions which treats dummy
entries (which have no baseline) little differently - the delta field
can be computed without baseline but others (ratio and wdiff) are not.

The new output will look like below:

  $ perf diff -o 2 perf.data.{old,cur,new}
  ...
  # Baseline/0  Delta/1  Delta/2  Shared Object      Symbol
  # ..........  .......  .......  .................  ..........................................
        22.98%   +0.51%   +0.52%  libc-2.20.so       [.] _int_malloc
         5.70%   +0.28%   +0.30%  libc-2.20.so       [.] free
         4.38%   -0.21%   +0.25%  a.out              [.] main
         1.32%   -0.15%   +0.05%  a.out              [.] free@plt
                          +0.01%  [kernel.kallsyms]  [k] intel_pstate_timer_func
                          +0.01%  [kernel.kallsyms]  [k] _raw_spin_lock_irqsave
                          +0.01%  [kernel.kallsyms]  [k] timekeeping_update.constprop.8
                 +0.01%   +0.01%  [kernel.kallsyms]  [k] apic_timer_interrupt
         0.01%            -0.00%  [kernel.kallsyms]  [k] native_read_msr_safe
         0.01%   -0.01%   -0.01%  [kernel.kallsyms]  [k] native_write_msr_safe
         1.31%   +0.03%   -0.06%  a.out              [.] malloc@plt
        31.50%   -0.74%   -0.23%  libc-2.20.so       [.] _int_free
        32.75%   +0.28%   -0.83%  libc-2.20.so       [.] malloc
         0.01%                    [kernel.kallsyms]  [k] scheduler_tick
                 +0.01%           [kernel.kallsyms]  [k] read_tsc
                 +0.01%           [kernel.kallsyms]  [k] perf_adjust_freq_unthr_context.part.82

In above example, the output was sorted by 'Delta/2' column first, and
then 'Baseline/0' and finally 'Delta/1'.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1420677949-6719-8-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>