[MERGE][1.8?] Make 'bzr co' more memory sensitive

[Andrew Bennetts]

John Arbash Meinel wrote:
[...]
> 
> If that is your only request, then I can submit the patch as is. (Note
> that I did work out a patch which adds an explicit test case, I just
> don't like it because it needs 100MB of ram and disk space, and takes
> 60s to finish.)

That sort of test probably belongs more in the usertest suite than “bzr
selftest”.

-Andrew.