[MERGE] ``BZR_LOG=disable`` suppresses writing messages to .bzr.log.

[Alexander Belchenko]

Bernhard Voelker пишет:
> why not let the OS decide what to do with it?
> I think
> 
>         _bzr_log_filename = '/dev/null'
> 
> is fully okay - every write to it will be automatically thrown
> away by the kernel.
> 
> With your solution, we'd make '/dev/null' and 'NUL' magic words,
> but from OS side they aren't! They're just character devices handled by 
> a device driver, in this case the sink.

May be you're right. I'll test it on Windows and then bb:resubmit.