Currently, we expose the function `git_repository_shallow_roots` to get
all grafted roots of the repository. This already paints us into a
corner, though, as we certainly need to experiment with some
functionality of the grafting mechanism before we can happily expose
some of its functionality. Most importantly, we need to get right when
to refresh grafts and when not.

Thus, this commit removes the public function with no public
replacement. We should first try and see what usecases people come up
with to e.g. expose the `git_grafts` mechanism directly in the future
or do something different altogether. Instead, we provide an internal
interface to get weak pointers to the grafting structs part of the
repository itself.