perf tools: Fix the bash completion for listing subcommands of perf

The bash completion does not support listing subcommands for 'perf
--<long option> <TAB>'.

Example:

Before this patch:

 $ perf --debug <TAB>
 $

As shown above, the subcommands of perf does not come out.

After this patch:

 $ perf --debug <TAB>
 annotate       buildid-cache  data           evlist         inject
 kvm            lock           probe          report         script
 test           top            version
 bench          buildid-list   diff           help           kmem
 list           mem            record         sched          stat
 timechart      trace

As shown above, the subcommands of perf can come out now.
Signed-off-by: Yunlong Song <yunlong.song@huawei.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1426685758-25488-7-git-send-email-yunlong.song@huawei.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/perf-completion.sh

@@ -130,7 +130,7 @@ __perf_main ()
 	# Skip options backward and find the last perf command
 	__perf_prev_skip_opts
 	# List perf subcommands or long options
-	if [ $cword -eq 1 ]; then
+	if [ -z $prev_skip_opts ]; then
 		if [[ $cur == --* ]]; then
 			cmds=$($cmd --list-opts)
 		else