When you add a new migration in any branch, run rake db:migrate and commit both the migration and db/schema.rb
If you do this, in development, you’ll be able to switch to another branch that has a different set of migrations and simply run rake db:schema:load.
Note that this will recreate the entire database, and existing data will be lost.
You’ll probably only want to run production off of one branch which you’re very careful with, so these steps don’t apply there (just run rake db:migrate as usual there). But in development, it should be no big deal to recreate the database from the schema, which is what rake db:schema:load will do.