BranchBuilder tests are not properly isolated

[John Arbash Meinel]

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I finally ran the whole test suite again on my machine, and I ran into a
couple bugs.

1) The bzrlib.branchbuilder doc tests do not isolate their environment.
Specifically, I have a default policy to sign all commits. And when
those tests run, it asks for my gpg key. I'm not really sure what to do
here, but it is annoying. (made more obvious because Hardy's gpg-agent
defaults to prompting for your approval before using a cached passphrase.)

2) The Dirstate logic about _read_link is failing for me, even on a
32-bit platform. Which really makes me wonder how it is working on PQM.
Is the test being skipped somehow? Maybe it just works on python2.4 and
not python2.5.

John
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