[MERGE] support updating lightweight checkouts of readonly branches

[John Arbash Meinel]

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Aaron Bentley wrote:
> Hi all,
> 
> This patch fixes the update code so that it works on lightweight
> checkouts of readonly branches.  This is probably most useful for
> same-location development.
> 
> For example, at work, it allows me to make checkouts of colleagues'
> work, and update them as my colleagues progress.
> 
> It also allows me to install bzr and bzrtools on my colleagues' machines
> as checkouts of my own bzr and bzrtools branches, so they can easily
> update to the latest version as appropriate.
> 
> Finally, it starts the work of disentangling the tree and branch
> updates.  Eventually, I expect _update_tree to become a public method
> that updates the tree to the latest branch revision, and to fix "bzr
> update" so that it handles conflicts better.
> 
> Aaron

I think this looks good, and is a small enough change to make it for
0.12. So go ahead and merge it.

I think Robert has a good idea about what direction we'd like to go. But
for now it makes lightweight checkouts of a readonly location useful.

John
=:->

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