Fix abort with nested Tree iteration

[rcoup]

When doing nested iteration of a Tree, Python crashes with an Abort from a libgit2 assert in git_tree_entry_byindex().

Code to reproduce:

for obj in some_tree:
    if isinstance(obj, pygit2.Tree):
        for obj2 in obj:
            pass

Fix is to ensure Trees are fully-loaded via Object__load() before creating a TreeIter, otherwise self->owner->tree in TreeIter_iternext() can resolve to NULL.