Add a new 'bup newliner' that fixes progress message whitespace.

Add a new 'bup newliner' that fixes progress message whitespace.

If we have multiple processes producing status messages to stderr and/or
stdout, and some of the lines ended in \r (ie. a progress message that was
supposed to be overwritten later) they would sometimes stomp on each other
and leave ugly bits lying around.

Now bup.py automatically pipes stdout/stderr to the new 'bup newliner'
command to fix this, but only if they were previously pointing at a tty.
Thus, if you redirect stdout to a file, nothing weird will happen, but if
you don't, stdout and stderr won't conflict with each other.

Anyway, the output is prettier now.  Trust me on this.