perf diff: Change default setting to "delta-abs"

The "delta-abs" compute method will show most changed entries on top.
So users can easily see how much effect between the data. Note that it
also changes the default of -o option to 1 in order to apply the compute
method. To see original-style (sorted by baseline) use -o 0 option.

Signed-off-by: Namhyung Kim <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/Documentation/perf-diff.txt

···
 -c::
 --compute::
         Differential computation selection - delta, ratio, wdiff, delta-abs
-        (default is delta).  Default can be changed using diff.compute
+        (default is delta-abs).  Default can be changed using diff.compute
         config option.  See COMPARISON METHODS section for more info.
 
 -p::
···
 -o::
 --order::
        Specify compute sorting column number.  0 means sorting by baseline
-       overhead (default) and 1 means sorting by computed value of column 1
+       overhead and 1 (default) means sorting by computed value of column 1
        (data from the first file other base baseline).  Values more than 1
        can be used only if enough data files are provided.
        The default value can be set using the diff.order config option.

tools/perf/builtin-diff.c

···
 static bool show_period;
 static bool show_formula;
 static bool show_baseline_only;
-static unsigned int sort_compute;
+static unsigned int sort_compute = 1;
 
 static s64 compute_wdiff_w1;
 static s64 compute_wdiff_w2;
···
 	[COMPUTE_WEIGHTED_DIFF] = "wdiff",
 };
 
-static int compute = COMPUTE_DELTA;
+static int compute = COMPUTE_DELTA_ABS;
 
 static int compute_2_hpp[COMPUTE_MAX] = {
 	[COMPUTE_DELTA]		= PERF_HPP_DIFF__DELTA,
···
 	OPT_BOOLEAN('b', "baseline-only", &show_baseline_only,
 		    "Show only items with match in baseline"),
 	OPT_CALLBACK('c', "compute", &compute,
-		     "delta,delta-abs,ratio,wdiff:w1,w2 (default delta)",
+		     "delta,delta-abs,ratio,wdiff:w1,w2 (default delta-abs)",
 		     "Entries differential computation selection",
 		     setup_compute),
 	OPT_BOOLEAN('p', "period", &show_period,