Forks and branches

Would be neat if there was a way to implement the concepts of forks and branches. The `.togepi` folder could store the required diff files for the different branches. A similar approach could be taken for forks.

The proposal mentions storing diff files in the .togepi folder, but names no implementation files, tests, or entry points. Start by reading the existing Python version-control and storage behavior, then clarify how branches differ from forks and what metadata each needs; done would mean both concepts can be created, stored, and used without ambiguity.

索引モデルが issue の本文から書いたものです。