perf diff: Fix to work with new hists design

The perf diff command is broken since:
  perf hists: Threaded addition and sorting of entries
  commit 1980c2ebd7020d82c024b8c4046849b38e78e7da

Several places were broken:
  - hists data need to be collected into opened sessions instead
    of into events
  - session's hists data need to be initialized properly when the
    session is created
  - hist_entry__pcnt_snprintf: the percentage and displacement
    buffer preparation must not use 'ret' because it's used
    as a pointer to the final buffer

Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20120322133726.GB1601@m.brq.redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
index 0221700..d9da62a 100644
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -34,7 +34,7 @@
 	return size;
 }
 
-static void hists__init(struct hists *hists)
+void hists__init(struct hists *hists)
 {
 	memset(hists, 0, sizeof(*hists));
 	hists->entries_in_array[0] = hists->entries_in_array[1] = RB_ROOT;