perf hists: Add a couple of hists stat helper functions

Add hists__{reset,inc}_[filter_]stats() functions to cleanup accesses
to hist stats (for output).  Note that number of samples in the stat
is not handled here since it belongs to the input stage.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/1398327843-31845-5-git-send-email-namhyung@kernel.org
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
diff --git a/tools/perf/builtin-diff.c b/tools/perf/builtin-diff.c
index c9cc771..52d91ac 100644
--- a/tools/perf/builtin-diff.c
+++ b/tools/perf/builtin-diff.c
@@ -573,10 +573,7 @@
 	hists->entries = RB_ROOT;
 	next = rb_first(root);
 
-	hists->nr_entries = 0;
-	hists->nr_non_filtered_entries = 0;
-	hists->stats.total_period = 0;
-	hists->stats.total_non_filtered_period = 0;
+	hists__reset_stats(hists);
 	hists__reset_col_len(hists);
 
 	while (next != NULL) {