[MERGE] Allow reference_revision to be None in the xml7 serializer.

[Jelmer Vernooij]

The attached patch allows TreeReference.reference_revision to be None
when serialized. Am I right thinking that None is an allowed value and
points to the latest current revision in the referenced branch?
It's being used as the default value in the constructor for
TreeReference. 

Cheers,

jelmer

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