[fastimport 3.2.0] AttributeError: 'LocalGitRepository' object has no attribute '_commit_builder_class'

Hi, I cannot fast-import in Breezy 3.2.0 a Git repository produced with:

```
git fast-export --all > project.fi
bzr fast-import project.fi project.bzr
```

...because the software crashes with the following error:

```[...]
File "~/.local/lib/python3.7/site-packages/breezy/plugins/fastimport/revision_store.py", line 249, in start_new_revision
    self._commit_builder = self.repo._commit_builder_class(self.repo,
AttributeError: 'LocalGitRepository' object has no attribute '_commit_builder_class'
```