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>

tools/perf/builtin-diff.c

@@ -18,6 +18,7 @@
 
 static char const *input_old = "perf.data.old",
 		  *input_new = "perf.data";
+static char	  diff__default_sort_order[] = "dso,symbol";
 static int  force;
 static bool show_displacement;
 
@@ -220,6 +221,7 @@ static const struct option options[] = {
 
 int cmd_diff(int argc, const char **argv, const char *prefix __used)
 {
+	sort_order = diff__default_sort_order;
 	argc = parse_options(argc, argv, options, diff_usage, 0);
 	if (argc) {
 		if (argc > 2)