[patch] Hashcache fix

[Aaron Bentley]

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Hi all,

I've fixed the hashcache.  It was a trivial fix; the WorkingTree
destructor was where the cache was written out, but that had been
deleted at some point.

In this version, I've hooked it up to WorkingTree.unlock.  I'd be happy
to hear a better suggestion, but at least this is deterministic.

Same old place:
http://panoramicfeedback.com/opensource/bzr/bzr.ab/

Aaron
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