Change message to "Your comment has been updated" on comment update

Description of the bug

We needed separate created and updated notifications for comments on a new Backdrop project. Unfortunately, the current state is that it always gives the "Your comment has been posted." message regardless if a comment was just created or updated. Nodes, on the other hand, have a separate message for update operation: "Post Your first post! has been updated.". So I believe this needs to be brought into consistency.

Additional information

This can be easily addressed by changing the following code in comment.module:

    if ($comment->status == COMMENT_NOT_PUBLISHED) {
      if (!user_access('administer comments')) {
        backdrop_set_message(t('Your comment has been queued for review by site administrators and will be published after approval.'));
      }
    }
    else {
      backdrop_set_message(t('Your comment has been posted.'));
    }

to

    if ($comment->status == COMMENT_NOT_PUBLISHED) {
      if (!user_access('administer comments')) {
        backdrop_set_message(t('Your comment has been queued for review by site administrators and will be published after approval.'));
      }
    }
    elseif (isset($comment->new)) {
      backdrop_set_message(t('Your comment has been updated.'));
    }
    else {
      backdrop_set_message(t('Your comment has been posted.'));
    }

and I'll be filing a PR soon. However, what I find it strange is that the $comment->new is set and equals to FALSE only on updates whereas on new comment creation it's not there at all. So I am not sure if this is a separate bug, because semantically it makes more sense to use an if condition something like this:

if ($comment->new == TRUE) {
  print created message here
} 
else {
  print updated message here
}

Let me know if we should chase why the $comment->new is not there for new comments or if the suggested PR is good enough.