new repo-wide config option for a "Binary Transparency" log based on git

A Binary Transparency Log is a append only log of all binaries published by a repo. This is useful for people to find whether the binary they have matches what F-Droid has published, and also makes it more difficult for the published history to be changed without notice, or for a server to give specific users custom malware binaries.

For a backstory, read:
https://www.eff.org/deeplinks/2014/02/open-letter-to-tech-companies

This also includes an option to generate fdroid verify output using diffoscope, if it is installed. This is only semi-related, but the code is totally isolated, so it seemed safe to include here.