perf hists browser: Count number of hierarchy entries

Add nr_hierarchy_entries field to keep current number of (unfolded) hist
entries.  And the hist_entry->nr_rows carries number of direct children.
But in the hierarchy mode, entry can have grand children and callchains.
So update the number properly using hierarchy_count_rows() when toggling
the folded state (by pressing ENTER key).

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/1456326830-30456-12-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>