- Use `association()` instead of `create()` in association blocks,
  to avoid enforcing the builder strategy.
- Pass on `project` in association blocks instead of `after` callbacks.
- Add `has_internal_id` trait to generate IIDs on stubbed models.
- Add `ProjectWiki#id` accessor.
- Rename `discussion_note_on_snippet` to `*_project_snippet`, for
  consistency with the `note_*` factories.
- Add `#sha` on stubbed commits in `RepoHelpers`.