This commit comes from the comments made by @matthewd at https://github.com/rails/rails/pull/23795/files#r54469637
and by @rafaelfranca at https://github.com/rails/rails/pull/23795/files#r54609364

The idea is that if you type (for example) "rake db:migrate" in an AppGenerator,
then this should actually invoke `rake db:migrate` on the command line, whereas
if you type "rails_command db:migrate", this should invoke `rails db:migrate`.