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

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 <[email protected]>
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>
Cc: Frédéric Weisbecker <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Paul Mackerras <[email protected]>
LKML-Reference: <1260979793[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>