Elasticsearch indexer attempts to reindex entire repository with pull mirror

If you set a project as a pull mirror, ElasticCommitIndexerWorker will be run without a from_sha and a to_sha, causing a full reindex of a repo: https://gitlab.com/gitlab-org/gitlab-ee/blob/v9.3.5-ee/app/models/ee/project/import_status_state_machine.rb#L50

For large repositories like the Linux repository, this can bring the system to its knees.