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

dirstate-v2: Enforce data size read from the docket file The data file may not be shorter than its size given by the docket. It may be longer, but additional data is ignored. Differential Revision: https://phab.mercurial-scm.org/D11089

dirstate-v2: Introduce a docket file .hg/dirstate now only contains some metadata to point to a separate data file named .hg/dirstate.{}.d with a random hexadecimal identifier. For now every update creates a new data file and removes the old one, but later we’ll (usually) append to an existing file. Separating into two files allows doing the "write to a temporary file then atomically rename into destination" dance with only a small docket file, without always rewriting a lot of data. Differential Revision: https://phab.mercurial-scm.org/D11088