PATCH: Automatically run new interleaved migrations

If you use Git for Rails development you know that now that migration
numbers are UTC timestamps, doing branchy development became a lot
easier. When merging, you just have to go back and find out which
migrations are new, and run them manually.

Or, with this patch (http://dev.rubyonrails.org/ticket/11493), you run
rake db:migrate, and Rails migrates (only) the new ones for you.
Please review the patch and +1 if you'd like to see it in Rails.