Revisioning: add test for assignment of revision labels

[jace]

What should the API be for assigning labels? Is it as simple as this:

rev.workflow_label = 'foo'

... which then automatically removes the label from previous uses, or should it be:

rev.workflow_label = 'foo'
oldrev.workflow_label = None

... which puts the onus on the caller, but making it possible to make mistakes. Or:

rev.assign_label('foo')

... which then finds the older revision and removes the label there. The difference from the first approach being that this does not override the lower level attribute assignment API.