Log Message:
Initial import of Avery Pennarun's redo implementation.

redo is a competitor to the long-lived, but sadly imperfect, make
program. Unlike other such competitors, redo captures the essential
simplicity and flexibility of make, while avoiding its flaws. It manages
to do this while being simultaneously simpler than make, more flexible
than make, and more powerful than make, and without sacrificing
performance -- a rare combination of features.

The original design for redo comes from Daniel J. Bernstein. This
from-scratch implementation of redo is based on that design.