Get the branching model config for a repository

Return the branching model configuration for a repository. The returned object:

1. Always has a development property for the development branch.
2. Always a production property for the production branch. The production branch can be disabled.
3. The branch_types contains all the branch types.
4. default_branch_deletion indicates whether branches will be deleted by default on merge.

This is the raw configuration for the branching model. A client wishing to see the branching model with its actual current branches may find the active model API more useful.