Migrations shouldn't fail RuboCop checks - especially lint checks, such
as the nested method check. To avoid changing code in existing
migrations, add the magic comment to the top of each of them to skip
that file.
Drop default_branch field from projects table
Use repository.root_ref as value for defautl_branch method
Fixes issue with default_branch and HEAD getting out of sync
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>