perf hists: Basic support of hierarchical report view

In the hierarchical view, entries will be grouped and sorted on the
first key, and then on the second key, and so on.  Add the
he->hroot_{in,out} fields to keep the lower level entries. Actually this
can share space, in a union, with callchain's 'sorted_root' since the
hroots are only used by non-leaf entries and callchain is only used by
leaf entries.

It also adds the 'parent_he' and 'depth' fields which can be used by browsers.

This patch only implements collapsing part which creates internal
entries for each sort key.  These need to be sorted by output_sort stage
and to be displayed properly in the later patch(es).

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Pekka Enberg <penberg@kernel.org>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1456326830-30456-3-git-send-email-namhyung@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>