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

[John Arbash Meinel]

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Jelmer Vernooij wrote:
| 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
|
|

As I understand it, None is only meant for working trees when you don't
know what the value is right now. For pre-dirstate trees, it would skip
the entry (it would omit the revision field in the xml text).  I'm
guessing it should do the same thing here, though I'm not positive.

John
=:->

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