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

[Andrew Bennetts]

Jonathan Lange wrote:
[...]
>      def overrideEnvironmentForTesting(self):
> +        old_home = os.environ['HOME']
> +        old_bzr_home = os.environ['BZR_HOME']
> +        def _restore_environment():
> +            os.environ['HOME'] = old_home
> +            os.environ['BZR_HOME'] = old_bzr_home
>          os.environ['HOME'] = self.test_home_dir
>          os.environ['BZR_HOME'] = self.test_home_dir
> +        self.addCleanup(_restore_environment)

You should use bzrlib.osutils.set_or_unset_env here.  See how
TestCase._cleanupEnvironment works for an example (and ideally perhaps re-use
TestCase._captureVar?).

Also, isn't the root problem really that:

    opaque_handle = enable_test_log(to_file)
    disable_test_log(opaque_handle)

isn't the no-op it should be?  Because it ends up effectively calling
enable_default_logging() instead of leaving the logging system uninitialised?

So I think perhaps a better fix would be to make enable/disable_test_log
actually *just* restore the state of the trace module, and not call
disable/enable_default_logging.

-Andrew.