[MERGE] BZR_LOG env variable controls location of .bzr.log.

[Robert Collins]

Robert Collins has voted resubmit.
Status is now: Resubmit
Comment:
I think that the trace.enable_default_logging() call should honour the 
new environment variable, for other programs using bzrlib.

also the new environ variable needs to be added to the list santised by 
the test suite.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C47AC0EC7.3000305%40ukr.net%3E