Create a tor-releases.git repository.

This repository should be a fork of the main Tor repository, containing only the releases. In particular, it should have:

• All supported tagged releases.
• "release-0.x.y" branches that are updated whenever a release is tagged, and not otherwise.
• "stable", "oldstable", "lts", and "alpha" branches that are updated whenever a release is tagged, and not otherwise. These branches may be force-pushed.
• A "master" branch containing only a README file that tells you what branches to really use.

For background, see https://www.mail-archive.com/tor-dev@lists.torproject.org/msg10246.html and the surrounding thread.

We should also update doc/HACKING/ReleasingTor.md to describe this process.