Textastic has ideal support for the iOS document picker, letting you pick files, directories and entire repositories. Changes you make in Textastic are immediately available in Working Copy and vice-versa.

Inside Textastic you use Open… to open files or Add External Folder… to open directories or repositories. Initially you need to enable Working Copy as a Location choice by picking … and More in the Files app.

You can read more about this in the Textastic documentation.