dirstate-v2: Move fixed-size tree metadata into the docket file Before this changeset, the dirstate-v2 data file contained not only nodes and paths that may be reused when appending to an existing file, but also some fixed-size metadata that applies to the entire tree and was added at the end of the data file for every append. This moves that metadata into the docket file, so that repeated "append" operations without meaningful changes don’t actually need to grow any file. Differential Revision: https://phab.mercurial-scm.org/D11098