[MERGE] Restore environment variables in a timely fashion (bug 124153)

[Robert Collins]

Robert Collins has voted -0.
Status is now: Waiting
Comment:
We already handle BZR_HOME and HOME in _cleanEnvironment. This is 
duplicative at best. There are many variables to handle properly, this 
test doesn't address the rest.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3Cd06a5cd30707082229w464d0aecjc5601364a98cba%40mail.gmail.com%3E