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