It's a follow up patch for a previous patch "perf tool: Return all
events as auto-completions after comma".

With this patch, auto-completion can work well for events with a ':'.
For example:

  root@skl:/tmp# perf stat -e block:block_<TAB>
  block:block_bio_backmerge   block:block_rq_complete
  block:block_bio_bounce      block:block_rq_insert
  block:block_bio_complete    block:block_rq_issue
  block:block_bio_frontmerge  block:block_rq_remap
  block:block_bio_queue       block:block_rq_requeue
  block:block_bio_remap       block:block_sleeprq
  block:block_dirty_buffer    block:block_split
  block:block_getrq           block:block_touch_buffer
  block:block_plug            block:block_unplug

  root@skl:/tmp# perf stat -e block:block_rq_<TAB>
  block:block_rq_complete  block:block_rq_issue     block:block_rq_requeue
  block:block_rq_insert    block:block_rq_remap

  root@skl:/tmp# perf stat -e block:block_rq_complete<TAB>
  block:block_rq_complete

  root@s...