Products.zopeversioncontrol

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- Updated package metadata.

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- Add omitted 'tests/common.py' module.

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- __init__.py, nonversioned.py: Fixed compatibility with Zope 2.8
and new-style objects (http://www.zope.org/Collectors/Zope/2137)

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- ZopeRepository.py: make ZR addable via GenericSetup toolset
(http://www.zope.org/Collectors/CMF/438).

- Utility.py: Import cleanup, including compatibility with ZODB 3.3+
location of 'refrencesf'.

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- IVersionControl.py: Added a module-scope alias for the benefit
of older software which depended on the old name.

- Hardened unit tests against the absence of the References product.

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- Refined the pattern for maintaining parts of objects independently
of version control. This is a generalization of the mechanism for
versioning container items. IVersionedContainer is now named
INonVersionedData and has more descriptive method names.

- 'updateResource' and 'uncheckoutResource' now retain the identity
of the object being versioned. That is, they never replace an
object with a new object, but instead change the state of an
existing object.

'updateResource' and 'uncheckoutResource' used to replace the
object in its container, but this strategy had two flaws:

1. It required ZopeVersionControl to use the ObjectManager API.
Version control should not require versionable objects to be
contained in ObjectManagers.

2. It assumes that versionable objects are simply wrapped using
acquisition. References (symlink-like objects) break this
assumption.