merge: change modified indicator to be 20 bytes Previously we indicated that the .hgsubstate file was dirty by adding a '+' to the end of its hash in the wctx manifest. This made is complicated to have new manifest implementations that rely on the node length being fixed. In previous patches we added added and modified node placeholders, so let's use those to indicate dirty here as well. It doesn't look like anything ever depended on this '+' (aside from it being different to the parent), so nothing else needed to change here.

dirstate: change added/modified placeholder hash length to 20 bytes Previously the added/modified placeholder hash for manifests generated from the dirstate was a 21byte long string consisting of the p1 file hash plus a single character to indicate an add or a modify. 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 these 20 bytes (just like we rely on no hash every being the nullid). This changes the possible behavior slightly in that the hash for all added/modified entries in the dirstate manifest will now be the same (so simple node comparisons would say they are equal), but we should never be doing simple node comparisons on these nodes even with the old hashes, because they did not accurately represent the content (i.e. two files based off the same p1 file node, with different working copy contents would have the same hash (even with the appended character) in the old scheme too, so we couldn't depend on the hashes period).

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.