dirstate-v2: Support appending to the same data file For now we’re still writing the entire data every time, so appending is not useful yet. Later we’ll have new nodes pointing to some existing data for nodes and paths that haven’t changed. The decision whether to append is pseudo-random in order to make tests exercise both code paths. This will be replaced by a heuristic based on the amount of unused existing data. Differential Revision: https://phab.mercurial-scm.org/D11094

dirstate-v2: shrink on-disk path lengths to 16-bits Differential Revision: https://phab.mercurial-scm.org/D11091

dirstate-v2: Rename Header to Root, move it to the end of the data file Now that they don’t have to be at the start, a given data file may contain multiple "roots". A docket only points to one of them, and previous ones are left unused to allow allow append-only in-place writing to an existing data file. Differential Revision: https://phab.mercurial-scm.org/D11090