perf hists: Fix children sort key behavior

perf hists: Fix children sort key behavior

When perf report --children resorts output fields, it tries to put
caller above the callee.  But this was only meaningful for a same thread
and doing this requires callchain enabled.  So fix its check before
comparing the callchain depth.

This also changes the hist accumulation tests: In test 3, xmalloc in
bash thread should be above than other perf threads due to alphabetical
order of comm string.  Also it's under page_fault in bash thread since
alphabetical order of dso name.  The sys_perf_event_open in perf thread
is put on the last line since it's self overhead is 0.

In test 4, the sys_perf_event_open is put above other perf entries that
have same children overhead since its callchain depth is smaller.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1419309381-2593-1-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>