perf diff: Change the default sort order to "dso,symbol"

This is a more intuitive / more meaningful default:

$ perf diff | head -8
     9.02%     +1.00%  libc-2.10.1.so               [.] _IO_vfprintf_internal
     2.91%     -1.00%  [kernel]                     [k] __kmalloc
     2.85%     -1.00%  [kernel]                     [k] ext4_htree_store_dirent
     1.99%     -1.00%  [kernel]                     [k] _atomic_dec_and_lock
     2.44%             [kernel]
$

Suggested-by: Ingo Molnar <mingo@elte.hu>
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: <1260979793-1981-1-git-send-email-acme@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>