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

[Ian Clatworthy]

Alexander Belchenko wrote:
> Here is another version.
> $BZR_LOG used to specify path for .bzr.log.
> If $BZR_LOG is /dev/null (Linux) or NUL (Windows) we are suppress log.

bb:tweak

The approach and code are fine IMO.

My only suggestion is that the one line of help added to
help_topics/__init__.py be expanded to 2 ala BZR_HOME. As the help
currently reads, a naive user might think that specifying /dev/null or
NUL suppresses the log, regardless of OS.

Ian C.