reindex in fast forwards can fail if we're changing non-model content.

```
ve/local/lib/python2.7/site-packages/elasticgit/workspace.py:156: in pull
    return self.reindex_diff(changes)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <elasticgit.workspace.Workspace object at 0x7f1fe5623f10>, diff_index = [<git.diff.Diff object at 0x7f1fe56bc578>, <git.diff.Diff object at 0x7f1fe56bc410>]

    def reindex_diff(self, diff_index):
        changed_model_set = set([])
        for diff in diff_index:
            if diff.new_file:
                changed_model_set.add(
>                   self.sm.path_info(diff.b_blob.path)[0])
E               TypeError: 'NoneType' object has no attribute '__getitem__'

ve/local/lib/python2.7/site-packages/elasticgit/workspace.py:123: TypeError
```