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

dirstate: replace a dead conditional branch with an assert in `update_file` This is a case we never meet, so lets trim it away to simplify the code before more changes. Differential Revision: https://phab.mercurial-scm.org/D11128