~bzr-pqm/bzr/bzr.dev

1311 by Martin Pool
- remove RevisionReference; just hold parent ids directly
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changes in weave branch:
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1313 by Martin Pool
- rename to Revision.parent_ids to avoid confusion with old usage
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 * Remove RevisionReference class; Revision.parent_ids is now simply a
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   list of their ids and parent_sha1s is a list of their corresponding
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   sha1s (for old branches only at the moment.)