perf help: Fix --help for builtins

perf help: Fix --help for builtins

It seems that commit cc5848213329 ("perf help: Remove use of die and
handle errors") caused the problem - it changed the initial value of
'help_format' from HELP_FORMAT_MAN to HELP_FORMAT_NONE.

This broke the --help option for all builtins, that would produce no
output, while 'man perf-top' would work it MANPATH is properly setup.

Reported-by: Stephane Eranian <eranian@google.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: David Ahern <dsahern@gmail.com>
Acked-by: David Ahern <dsahern@gmail.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/87r4orj7zc.fsf@sejong.aot.lge.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>