With debug logging level set long running commands do not show their output in real time

I've seen this problem on FreeBSD and I've been able to duplicate it on Mac OS X as well but I don't know about other platforms.

Long running commands (such as installing via ports in FreeBSD) do not show the output logged to standard out as it's created. Rather, they output all at once, only when the command has finished. This isn't much of a problem for short-running commands but it can be a big problem for commands that take a while to run (such as installing packages, especially from source).

Obviously, this is to do with buffering.

I have some fixes (for the run_command mixin) that address the issue on FreeBSD but I won't add the patches here until I manage to write some unit tests that show up the issue...