[MERGE] loosen the constraints on WorkingTree.set_last_revision.

[Aaron Bentley]

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Robert Collins wrote:
> On Mon, 2006-08-07 at 19:07 -0400, Aaron Bentley wrote:
>> Setting last_revision to a ghost we no longer consider it illegal for
>> a
>> working tree to refer to a revision not present in a branch.  Which
>> means it's no longer safe to purge revisions from a repo when they're
>> not referenced by the branches it contains. 
> 
> Huh? That does not follow. If the last revision of a tree *may* be a
> ghost has no bearing on the policy for removing data from a repository -
> that policy is still (AFAIK) 'only remove data that no *branches* refer
> to.

That's only legitimate if having a last_revision not in the branch is an
illegal state.  I'm not saying 'unreachable' here, but 'illegal'.  If
it's not an illegal state, then you can destroy legitimate working trees
with clean-my-repo, and that's very bad.

Aaron
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