[MERGE] add lock hooks

[Robert Collins]

Robert Collins has voted resubmit.
Status is now: Resubmit
Comment:
John, Martin has invented a new idiom for using Hooks, via his clone and 
clear_hooks() methods. These are untested - so for my part, this is 
tweak() - if we're going to invent a new approach, it should be tested. 
(The difference is that rather than constructing a new PhysicalLockHooks 
instance, he's reusing the existing one.

I don't like this because it isn't as isolated, and you can't test the 
hooks independently because they no longer have solid isolation. I'd 
rather we keep the previous approach.

For details, see: 
http://bundlebuggy.aaronbentley.com/project/bzr/request/%3Ce01316480805262027ifd95757qec09f83954c3019c%40mail.gmail.com%3E
Project: Bazaar