Example / streamline API for retrieving file contents

While working on [tdlib-versioned](https://github.com/ForNeVeR/tdlib-versioned) (currently non-public, will be published in a couple of days), I encountered a couple of problems: there's not enough documentation and no API to do basic tasks, like enumerate the Git commit files and extract their contents by path.

We should add an example on how to do that, and streamline the API for that purpose.