That way, the head is static which allows to re-use the worktree status across
multiple related calls.
Also, add a way to quickly get the `head_commit()` from a `git2::Repository`.
Note that this may cause breakage in the app, as now it will see the actual
head, not a computed one, but that should also help us to find places that
didn't properly update their state.
The worktree diff is essentially a 'git status' with a given tree,
and that can be expressed with a diff of the workdir, the index and a tree.
This comes at the expense of not being able to control which file sizes are diffed.
