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