ALTER TABLE comments MODIFY COLUMN repo_name varchar(255) NULL;
ALTER TABLE pull_labels MODIFY COLUMN repo_name varchar(255);
ALTER TABLE pulls MODIFY COLUMN repo_name varchar(255);

ALTER TABLE pulls DROP KEY pulls_repo;
ALTER TABLE pulls ADD KEY pulls_repo (repo);

Allow null values in repo_name so code can be removed
Allow null values in repo_name so code can be removed

Code that references repo_name can be removed after this migration is
ran so that when the ORM tries to insert, mysql will fill all the values
with null since they won't be provided by the code.

Since repo_name is no longer going to be used, repo should have the
index.
