perf hists browser: Fix percent display in callchains

When there's only a single callchain, perf doesn't print its percentage
in front of the symbols.  This is because it assumes that the percentage
is same as parents.  But if a percent limit is applied, it's possible
that there are actually a couple of child nodes but only one of them is
shown.  In this case it should display the percent to prevent
misunderstanding of its percentage is same as the parent's.

For example, let's see the following callchain.

  $ perf report --no-children --percent-limit 0.01 --tui
  ...
  -    0.06%  sleep    [kernel.vmlinux]    [k] kmem_cache_alloc_trace
       kmem_cache_alloc_trace
     - perf_event_mmap
        - 0.04% mmap_region
             do_mmap_pgoff
           - vm_mmap_pgoff
              + 0.02% sys_mmap_pgoff
              + 0.02% vm_mmap
           + 0.02% mprotect_fixup

Current code omits the percent if 'mmap_region' becomes the only node
when percent limit is set to 0.03%, its percent is not 0.06% but users
will assume it incorrectly.

Before:

  $ perf report --no-children --percent-limit 0.03 --tui
  ...
     0.06%  sleep    [kernel.vmlinux]    [k] kmem_cache_alloc_trace
       kmem_cache_alloc_trace
     - perf_event_mmap
        - mmap_region
          do_mmap_pgoff
          vm_mmap_pgoff

After:

  $ perf report --no-children --percent-limit 0.03 --tui
  ...
     0.06%  sleep    [kernel.vmlinux]    [k] kmem_cache_alloc_trace
       kmem_cache_alloc_trace
     - perf_event_mmap
        - 0.04% mmap_region
             do_mmap_pgoff
             vm_mmap_pgoff

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1453909257-26015-10-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>