Add entity: Tree

[kondziu]

People often ask us about the files contained in the repository currently or at a particular time. The contents of a repository at a specific time are called a tree. It should be equivalent to what you get when you clone a repository, and explore through the files, basically.

One can create a tree from any commit, by retracing its history.

We would not want to have Djanco calculate all trees for all commits and we do not want them hanging around, so they need to be calculated just as needed.