code: func NewLocalGitSource(gitDir, file, commitish string) SchemaSource {
  	return &localGitSource{
  		dir:       gitDir,
  		file:      file,
  		commitish: commitish,
  	}
  }
docs: // NewLocalGitSource creates a SchemaSource whose contents are derived from
  // the given file at the given commit ID in a git repository.