perf: Allow comparing perf reports with only one difference.

The cairo-perf-diff-files tool would ignore perf reports with
just one test for no apparent reason.  The traces take so long
that it's useful to be able to compare runs with just one trace.

perf/cairo-perf-diff-files.c

@@ -314,7 +314,7 @@ cairo_perf_reports_compare (cairo_perf_report_t		*reports,
 	diff++;
 	num_diffs++;
     }
-    if (num_diffs < 2)
+    if (num_diffs == 0)
 	goto DONE;
 
     if (num_reports == 2)