perf evlist: Print hint for group

An event group is a critical relationship. There is a -g option that can
display the relationship. But it's hard for a user to know when should
this option be applied.

If there is an event group in the perf record, print a hint to suggest
the user apply the -g to display the group information.

With the patch,

  $ perf record -e "{cycles,instructions},instructions" sleep 1
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.024 MB perf.data (4 samples) ]
  $

  $ perf evlist
  cycles
  instructions
  instructions
  # Tip: use 'perf evlist -g' to show group information

  $ perf evlist -g
  {cycles,instructions}
  instructions
  $

Committer testing:

So for a perf.data file _with_ a group:

  root@number:~# perf evlist -g
  {cpu_core/branch-instructions/pp,cpu_core/branches/}
  dummy:u
  root@number:~# perf evlist
  cpu_core/branch-instructions/pp
  cpu_core/branches/
  dummy:u
  # Tip: use 'perf evlist -g' to show group information
  root@number:~#

Then for something _without_ a group, no hint:

  root@number:~# perf record ls
  <SNIP>
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.035 MB perf.data (7 samples) ]
  root@number:~# perf evlist
  cpu_atom/cycles/P
  cpu_core/cycles/P
  dummy:u
  root@number:~#

No suggestion, good.

Suggested-by: Arnaldo Carvalho de Melo <[email protected]>
Reviewed-by: Ian Rogers <[email protected]>
Signed-off-by: Kan Liang <[email protected]>
Tested-by: Arnaldo Carvalho de Melo <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Namhyung Kim <[email protected]>
Closes: https://lore.kernel.org/lkml/ZttgvduaKsVn1r4p@x1/
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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