treemanifest: speed up diff by keeping track of dirty nodes Since tree manifests have a nodeid per directory, we can avoid diffing entire directories if they have the same nodeid. The comparison is only valid for unmodified treemanifest instances, of course, so we need to keep track of which have been modified. Therefore, let's add a dirty flag to treemanifest indicating whether its nodeid can be trusted. We set it when _files or _dirs is modified, and make diff(), and its cousin filesnotin(), not descend into subdirectories that are the same on both sides. On the Mozilla repo, this speeds up 'hg diff -r .^ -r .' from 1.990s to 1.762s. The improvement will be much larger when we start lazily loading subdirectory manifests.