perf hist: Initialize hierarchy tree explicitly

The hroot_in and hroot_out are roots of hierarchy trees of hist entries.

But when a hist entry is initialized by copying existing template entry,
it sometimes has non-empty tree and copies it incorrectly.  This is a
problem especially when an event group is used since it creates dummy
entries from already-processed entries in other event members.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20160913074552.13284-4-namhyung@kernel.org


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