perf tools: Add the bash completion for listing subsubcommands of perf help

The bash completion does not support listing subsubcommands for 'perf
help <TAB>', so fix it.

Example:

Before this patch:

 $ perf help <TAB>
 $

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

After this patch:

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

As shown above, the subsubcommands of perf help 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/1426685758-25488-9-git-send-email-yunlong.song@huawei.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>