Add `flatten_completions` to igor_common.
Add `flatten_completions` to igor_common.

uwsampa/grappa,buaasun/grappa,alexfrolov/grappa,buaasun/grappa,alexfrolov/grappa,buaasun/grappa,uwsampa/grappa,buaasun/grappa,uwsampa/grappa,alexfrolov/grappa,alexfrolov/grappa,alexfrolov/grappa,buaasun/grappa,alexfrolov/grappa,uwsampa/grappa,buaasun/grappa,buaasun/grappa,uwsampa/grappa,uwsampa/grappa,uwsampa/grappa,alexfrolov/grappa