perf diff: Fix support for all --sort combinations

perf diff: Fix support for all --sort combinations

When we finish creating the hist_entries we _already_ have them
sorted "by name", in fact by what is in --sort, that is exactly
how we can find the pairs in perf_session__match_hists as
'comm', 'dso' & 'symbol' all are strings we need to find the
matches in the baseline session.

So only do the sort by hits followed by a resort by --sort if we
need to find the position for shwowing the --displacement of
hist entries.

Now all these modes work correctly:

Example is a simple 'perf record -f find / > /dev/null' ran
twice then followed by the following commands:

  $ perf diff -f --sort comm
  # Baseline  Delta      Command
  # ........ ..........  .......
  #
       0.00%   +100.00%     find
  $ perf diff -f --sort dso
  # Baseline  Delta           Shared Object
  # ........ ..........  ..................
  #
      59.97%     -0.44%  [kernel]
      21.17%     +0.28%  libc-2.5.so
      18.49%     +0.16%  [ext3]
       0.37%             find
  $ perf diff -f --sort symbol | head -8
  # Baseline  Delta      Symbol
  # ........ ..........  ......
  #
       6.21%     +0.36%  [k] ext3fs_dirhash
       3.43%     +0.41%  [.] __GI_strlen
       3.53%     +0.16%  [k] __kmalloc
       3.17%     +0.49%  [k] system_call
       3.06%     +0.37%  [k] ext3_htree_store_dirent
  $ perf diff -f --sort dso,symbol | head -8
  # Baseline  Delta           Shared Object  Symbol
  # ........ ..........  ..................  ......
  #
       6.21%     +0.36%  [ext3]              [k] ext3fs_dirhash
       3.43%     +0.41%  libc-2.5.so         [.] __GI_strlen
       3.53%     +0.16%  [kernel]            [k] __kmalloc
       3.17%     +0.49%  [kernel]            [k] system_call
       3.06%     +0.37%  [ext3]              [k] ext3_htree_store_dirent
  $

And we don't have to do two expensive resorts in the common, non
--displacement case.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <1262047716-23171-5-git-send-email-acme@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>