dirstate: change placeholder hash length to 20 bytes Previously the new-node placeholder hash for manifests generated from the dirstate was a 21byte long string of "!" characters. Normal hashes are only 20 bytes long. This makes it complicated to implement more efficient manifest implementations which rely on the hashes being fixed length. Let's change this hash to just be 20 bytes long, and rely on the astronomical improbability of an actual hash being 20 "!" bytes in a row (just like we rely on no hash ever being the nullid). A future diff will do this for added and modified dirstate markers as well, so we're putting the new newnodeid in node.py so there's a common place for these placeholders.