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>

tools/perf/builtin-help.c

@@ -414,7 +414,7 @@ static int show_html_page(const char *perf_cmd)
 int cmd_help(int argc, const char **argv, const char *prefix __maybe_unused)
 {
 	bool show_all = false;
-	enum help_format help_format = HELP_FORMAT_NONE;
+	enum help_format help_format = HELP_FORMAT_MAN;
 	struct option builtin_help_options[] = {
 	OPT_BOOLEAN('a', "all", &show_all, "print all available commands"),
 	OPT_SET_UINT('m', "man", &help_format, "show man page", HELP_FORMAT_MAN),