Fill in percentage in file status lines.

Fill in percentage in file status lines.

I was putting that off because I had a half-formed clever idea about
trying to do linear regression on the posts we _have_ formatted to the
current width, and using it to estimate the likely line height of the
ones we haven't. But then I decided that was either the perfect being
the enemy of the good, or a silly idea that was bound to explode in an
edge case, or both, and so I've done something simple instead.

The _main_ purpose is that '100%' means 'nothing more to read' and
anything less means there is. As long as we get that much right, we're
in reasonably good shape.