* KnitIndex.get_parents now returns tuples. (Robert Collins)
INTERNALS:
* Unused functions on the private interface KnitIndex have been removed. (Robert Collins)
* New ``knit.KnitGraphIndex`` which provides a ``KnitIndex`` layered on top of a ``index.GraphIndex``. (Robert Collins)
* New ``knit.KnitVersionedFile.iter_parents`` method that allows querying the parents of many knit nodes at once, reducing round trips to the underlying index. (Robert Collins)