perf diff: Print diff result more precisely

Current perf diff result is somewhat confusing since it sometimes hide
small result and sometimes there's no result.  So do not hide small
result (less than 0.01%) and print "N/A" if baseline is not
recorded (for ratio and wdiff only).  Blank means the baseline is
available but its pairs are not.

Before:

  # Baseline    Delta  Shared Object      Symbol
  # ........  .......  .................  .........................
  #
       ...
       0.01%   -0.01%  [kernel.kallsyms]  [k] native_write_msr_safe
       0.01%           [kernel.kallsyms]  [k] scheduler_tick
       0.01%           [kernel.kallsyms]  [k] native_read_msr_safe
       0.00%           [kernel.kallsyms]  [k] __rcu_read_unlock
                       [kernel.kallsyms]  [k] _raw_spin_lock
               +0.01%  [kernel.kallsyms]  [k] apic_timer_interrupt
                       [kernel.kallsyms]  [k] read_tsc

After:

  # Baseline    Delta  Shared Object      Symbol
  # ........  .......  .................  .........................
  #
       ...
       0.01%   -0.01%  [kernel.kallsyms]  [k] native_write_msr_safe
       0.01%           [kernel.kallsyms]  [k] scheduler_tick
       0.01%           [kernel.kallsyms]  [k] native_read_msr_safe
       0.00%           [kernel.kallsyms]  [k] __rcu_read_unlock
               +0.01%  [kernel.kallsyms]  [k] _raw_spin_lock
               +0.01%  [kernel.kallsyms]  [k] apic_timer_interrupt
               +0.01%  [kernel.kallsyms]  [k] read_tsc

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: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1419656793-32756-3-git-send-email-namhyung@kernel.org


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