Regression: enumerating pygit2.Tree with submodules raises KeyError

```
$ git clone https://git.sr.ht/~sircmpwn/drewdevault.com
$ cd drewdevault.com
$ python3
>>> from pygit2 import Repository
>>> repo = Repository(".")
>>> c = repo.get(repo.head.target)
>>> list(c.tree)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'object not found - no match for id (a677dcf495c1973ba11a74d65676a003224d44c4)'
```