[RFC] Dirstate blowing up on None as a minikind fingerprint.

[Wouter van Heyst]

Hey all,

on the nested-trees branch a test was failing with funny dirstate
errors, it turns out because the reference_revision for the entry is
None and that got fed into DirState.save() which proceeded to blow up.

The attached fix gets rid of that, but it's not directly tested
(TestCompositeTree.test_apply_inventory_delta does indirectly).

Wouter van Heyst