Fix "Migrate status with empty --tag doesn't work"

[claudiu-cristea]

[weitzman]

I dont think we do grouping within a table anywhere else in Drush. We used to, and it got awkward. What we do is use a default output format of yaml instead of table. Maybe it is less scannable, but it supports grouping well. For example see drush role:list, drush site:alias. I'm not 100% against using tables with if everyone wants that here.