perf diff: Make diff command work with evsel hists

Putting 'perf diff' command back on track with the 'latest'
evsel hists changes. Each evsel has its own 'hists' object
gathering stats for the particular event.

While currently counts are accumulated for the whole session
regardless of the events diversification within compared
sessions.

The 'perf diff' command now outputs all matching events within
compared sessions (with event name specified). The per event
diff output stays the same.

  $ ./perf diff
  # Event 'cycles'
  #
  # Baseline  Delta          Shared Object                          Symbol
  # ........ ..........  .................  ..............................
  #
       0.00%    +15.14%  [kernel.kallsyms]  [k] __wake_up
       0.00%    +13.38%  [kernel.kallsyms]  [k] ext4fs_dirhash

... SNIP

       0.00%     +0.42%  [kernel.kallsyms]  [k] local_clock
       0.17%     -0.05%  [kernel.kallsyms]  [k] native_write_msr_safe

  # Event 'faults'
  #
  # Baseline  Delta          Shared Object                          Symbol
  # ........ ..........  .................  ..............................
  #
       0.00%    +79.12%  ld-2.15.so         [.] _dl_relocate_object
       0.00%    +11.62%  ld-2.15.so         [.] openaux

Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1346946426-13496-2-git-send-email-jolsa@redhat.com


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