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

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

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

Example:

Before this patch:

 $ perf script <TAB>
 $

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

After this patch:

 $ perf script <TAB>
 record  report

As shown above, the subsubcommands of perf script 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/1426685758-25488-10-git-send-email-yunlong.song@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>