We add tests for ODB functionality, as well as a new `TreeBuilder`
Struct to make building git trees easier (see below).
The `git2::TreeBuilder` can be a pain to use since it has a lifetime
that is tied to the `git2::Repository`. This was causing issues when
trying to write generators for property tests.
Instead, define a custom TreeBuilder which is a defunctionalised list
operations that represent the `insert` and `remove` methods on
`git2::TreeBuilder`.
The implementation of writing a tree then becomes converting the
custom `TreeBuilder` into a `git2::TreeBuilder`.
Signed-off-by: Jorge Iglesias Garcia <me@jorgeig.com>
