This change makes the DiffNotifyCallback always use an "unowned"
*git.Diff that does _not_ run the finalizer. Since the underlying
git_diff object is still owned by libgit2, we shouldn't be calling
Diff.Free() on it, even by accident, since that would cause a whole lot
of undefined behavior.
Now instead of storing a reference to a *git.Diff in the intermediate
state while the diff operation is being done, create a brand new
*git.Diff for every callback invocation, and only create a fully-owned
*git.Diff when the diff operation is done and the ownership is
transfered to Go.
lhchavez