perf diff: Fix to sort by baseline field by default

The currently perf diff didn't add the baseline and delta (or other
compute) fields to the sort list so output will be sorted by other
fields like alphabetical order of DSO or symbol as below example.

Fix it by adding hpp formats for the fields and provides default compare
functions.

Before:

  $ perf diff
  # Event 'cycles'
  #
  # Baseline    Delta  Shared Object       Symbol
  # ........  .......  ..................  ...............................
  #
                       [bridge]            [k] ip_sabotage_in
                       [btrfs]             [k] __etree_search.constprop.47
       0.01%           [btrfs]             [k] btrfs_file_mmap
       0.01%   -0.01%  [btrfs]             [k] btrfs_getattr
                       [e1000e]            [k] e1000_watchdog
       0.00%           [kernel.vmlinux]    [k] PageHuge
       0.00%           [kernel.vmlinux]    [k] __acct_update_integrals
       0.00%           [kernel.vmlinux]    [k] __activate_page
                       [kernel.vmlinux]    [k] __alloc_fd
       0.02%   +0.02%  [kernel.vmlinux]    [k] __alloc_pages_nodemask
       ...

After:

  # Baseline    Delta  Shared Object       Symbol
  # ........  .......  ..................  ................................
  #
      24.73%   -4.62%  perf                [.] append_chain_children
       7.96%   -1.29%  perf                [.] dso__find_symbol
       6.97%   -2.07%  libc-2.20.so        [.] vfprintf
       4.61%   +0.88%  libc-2.20.so        [.] __fprintf_chk
       4.41%   +2.43%  perf                [.] sort__comm_cmp
       4.10%   -0.16%  perf                [.] comm__str
       4.03%   -0.93%  perf                [.] machine__findnew_thread_time
       3.82%   +3.09%  perf                [.] __hists__add_entry
       2.95%   -0.18%  perf                [.] sort__dso_cmp
       ...

Signed-off-by: Namhyung Kim <namhyung@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/1419656793-32756-1-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>