perf diff: Fix output ordering to honor next column

When perf diff prints output, it sorts the entries using baseline field
by default, but entries which don't have baseline are not sorted
properly.  This patch makes it sorted by values of next column.

Before:

  # Baseline/0  Delta/1  Delta/2  Shared Object      Symbol
  # ..........  .......  .......  .................  ..........................................
  #
        32.75%   +0.28%   -0.83%  libc-2.20.so       [.] malloc
        31.50%   -0.74%   -0.23%  libc-2.20.so       [.] _int_free
        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
         1.31%   +0.03%   -0.06%  a.out              [.] malloc@plt
         0.01%   -0.01%   -0.01%  [kernel.kallsyms]  [k] native_write_msr_safe
         0.01%                    [kernel.kallsyms]  [k] scheduler_tick
         0.01%            -0.00%  [kernel.kallsyms]  [k] native_read_msr_safe
                          +0.01%  [kernel.kallsyms]  [k] _raw_spin_lock_irqsave
                 +0.01%   +0.01%  [kernel.kallsyms]  [k] apic_timer_interrupt
                          +0.01%  [kernel.kallsyms]  [k] intel_pstate_timer_func
                 +0.01%           [kernel.kallsyms]  [k] perf_adjust_freq_unthr_context.part.82
                 +0.01%           [kernel.kallsyms]  [k] read_tsc
                          +0.01%  [kernel.kallsyms]  [k] timekeeping_update.constprop.8

After:

  # Baseline/0  Delta/1  Delta/2  Shared Object      Symbol
  # ..........  .......  .......  .................  ..........................................
  #
        32.75%   +0.28%   -0.83%  libc-2.20.so       [.] malloc
        31.50%   -0.74%   -0.23%  libc-2.20.so       [.] _int_free
        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
         1.31%   +0.03%   -0.06%  a.out              [.] malloc@plt
         0.01%   -0.01%   -0.01%  [kernel.kallsyms]  [k] native_write_msr_safe
         0.01%                    [kernel.kallsyms]  [k] scheduler_tick
         0.01%            -0.00%  [kernel.kallsyms]  [k] native_read_msr_safe
                 +0.01%   +0.01%  [kernel.kallsyms]  [k] apic_timer_interrupt
                 +0.01%           [kernel.kallsyms]  [k] read_tsc
                 +0.01%           [kernel.kallsyms]  [k] perf_adjust_freq_unthr_context.part.82
                          +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

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/1420677949-6719-7-git-send-email-namhyung@kernel.org
[ Fixed up hist_entry__cmp_ method signatures, fallout from making previous cset buildable ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>