perf hists browser: Align column header in hierarchy mode

perf hists browser: Align column header in hierarchy mode

Like in stdio, fit column header to hierarchy output.  Merge column
headers with "/" as a separator.

   Overhead        Command / Shared Object / Symbol
  ...
  +   0.09%        dwm
  +   0.06%        emacs
  -   0.05%        perf
     -   0.05%        [kernel.vmlinux]
        +   0.03%        [k] memcpy_orig
        +   0.01%        [k] unmap_single_vma
        +   0.01%        [k] smp_call_function_single
        +   0.00%        [k] native_irq_return_iret
        +   0.00%        [k] arch_trigger_all_cpu_backtrace_handler
        +   0.00%        [k] native_write_msr_safe

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/1456326830-30456-15-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>