tools lib subcmd: Show parent options in help

I've just realized that help message in a subcommand didn't show one
in the parent command.  Since the option parser understands the parent,
display code should do the same.  For example, `perf ftrace latency -h`
should show options in the `perf ftrace` command too.

Before:

  $ perf ftrace latency -h

   Usage: perf ftrace [<options>] [<command>]
      or: perf ftrace [<options>] -- [<command>] [<options>]
      or: perf ftrace {trace|latency} [<options>] [<command>]
      or: perf ftrace {trace|latency} [<options>] -- [<command>] [<options>]

      -b, --use-bpf         Use BPF to measure function latency
      -n, --use-nsec        Use nano-second histogram
      -T, --trace-funcs <func>
                            Show latency of given function

After:

  $ perf ftrace latency -h

   Usage: perf ftrace [<options>] [<command>]
      or: perf ftrace [<options>] -- [<command>] [<options>]
      or: perf ftrace {trace|latency} [<options>] [<command>]
      or: perf ftrace {trace|latency} [<options>] -- [<command>] [<options>]

      -a, --all-cpus        System-wide collection from all CPUs
      -b, --use-bpf         Use BPF to measure function latency
      -C, --cpu <cpu>       List of cpus to monitor
      -n, --use-nsec        Use nano-second histogram
      -p, --pid <pid>       Trace on existing process id
      -T, --trace-funcs <func>
                            Show latency of given function
      -v, --verbose         Be more verbose
          --tid <tid>       Trace on existing thread id (exclusive to --pid)

Reviewed-by: Ian Rogers <[email protected]>
Signed-off-by: Namhyung Kim <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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