(Copied from http://reviews.review-board.org/r/937/)

I've had a few cases where it's more convenient to just build the diff myself based on just a select number of
files, or perhaps cleanup through another script. Seemed simple enough to support piping into postreview.

Been using this patch against an older version for a few months, but it would be easier to just have it included upstream.

Used it to create this review.