Support early cutoff optimization

[obiwac]

Using the example (Fig. 3) in Build Systems à la Carte: Theory and Practice, where a source file only has a comment added which doesn't change the output, don't trigger a re-link (Shake supports this).

In practice I don't really know how this could be done efficiently without having to outright compare hashes, but maybe that performance hit is marginal compared to the compilation anyway. Also we'd have to convert Bob to be completely self-tracking (cf. §5.1).