perf tools: Return all events as auto-completions after comma

It's a follow up for one previous patch "perf tool: Improve bash command
line auto-complete for multiple events with comma."

It fixes an issue that no events are displayed when <TAB> is directly
typed after comma.

With this patch, now the result is:

  root@skl:/tmp# perf stat -e cpu-cycles,<TAB>
  Display all 2389 possibilities? (y or n)
  alarmtimer:alarmtimer_cancel
  alarmtimer:alarmtimer_fired
  alarmtimer:alarmtimer_start
  alarmtimer:alarmtimer_suspend
  alignment-faults
  arith.divider_active
  BAClear_Cost
  baclears.any
  block:block_bio_backmerge
  block:block_bio_bounce
  block:block_bio_complete
  block:block_bio_frontmerge
  block:block_bio_queue
  block:block_bio_remap
  block:block_dirty_buffer
  block:block_getrq
  block:block_plug
  block:block_rq_complete
  block:block_rq_insert
  block:block_rq_issue
  block:block_rq_remap
  block:block_rq_requeue
  block:block_sleeprq
  --More--

One remaining issue is that the auto-completions doesn't work well
for the event with ':'. For example, clk:clk_enable.

Because ':' is set as WORDBREAK by default in bash. Need more work
for this case.
Reported-by: NArnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: NJin Yao <yao.jin@linux.intel.com>
Tested-by: NArnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1513940255-16528-1-git-send-email-yao.jin@linux.intel.comSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/perf-completion.sh

@@ -183,12 +183,16 @@ __perf_main ()
 				fi
 			done
 
-			evts=${result}+$(ls /sys/bus/event_source/devices/cpu/events)
+			evts=${result}" "$(ls /sys/bus/event_source/devices/cpu/events)
 		else
-			evts=${raw_evts}+$(ls /sys/bus/event_source/devices/cpu/events)
+			evts=${raw_evts}" "$(ls /sys/bus/event_source/devices/cpu/events)
 		fi
 
-		__perfcomp_colon "$evts" "$cur1"
+		if [[ "$cur1" == , ]]; then
+			__perfcomp_colon "$evts" ""
+		else
+			__perfcomp_colon "$evts" "$cur1"
+		fi
 	else
 		# List subcommands for perf commands
 		if [[ $prev_skip_opts == @(kvm|kmem|mem|lock|sched|