Self-referential repo + tree

I've found that I often need to store a Repository and a Tree<'repo> that borrows from it. Unfortunately, this creates unsafe self-referential structs.

It would be helpful if this crate provided a safe abstraction for repo + tree together, e.g.

let repo = git2::Repository::open()?;

/// makes RepositoryWithTree
let repo = repo.with_tree(|repo| {
	repo.find_commit(head)?.tree()
});

*repo // Derefs to Repository
repo.tree(); // returns borrowed Tree

Alternatively, there's owning-ref that has OwningHandle trait. It would require git2 types to implement its traits.

Start with the Repository and Tree APIs and the proposed with_tree entry point; compare the owning_ref OwningHandle requirements mentioned in the issue. Define the safe ownership and borrowing behavior, then validate it with focused tests; done means callers can keep a Repository and borrowed Tree together without unsafe self-referential structs.