Open or create a Git repositorySignature:
Create a new Git repository if a dbName specified in the constructor does not exist.
GitDocumentDB creates a legitimate Git repository and unique metadata under '.gitddb/'.
'.gitddb/' keeps DatabaseInfo for combining databases, checking schema and migration.
GitDocumentDB can also load a Git repository that is created by other apps. It almost works; however, correct behavior is not guaranteed if it does not have a valid '.gitddb/'.
Err.RepositoryNotFoundError may occurs when openOptions.createIfNotExists is false.
# Errors from _createRepository
# Errors from putWorker