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

dirstate-v2: Add heuristic for when to create a new data file … instead of appending to the existing one. This is based on keeping track of how much of the existing data is not used anymore. Differential Revision: https://phab.mercurial-scm.org/D11097

dirstate-v2: Reuse existing paths when appending to a data file When writing a dirstate in v2 format by appending to an existing data file, filenames / paths that are borrowed from the previous on-disk representation can be reused. Differential Revision: https://phab.mercurial-scm.org/D11096