dirstate-v2: Swap the order of size and mtime on disk This makes the dirstate-v2 file format match dirstate-v1 for the order of `mode`, `size`, and `mtime`. This order does not matter as long as these components are handled through named fields/attributes in code, but in a few places we still have tuples so having the same order everywhere might help avoid a bug that might not be obvious since those components have the same type. Differential Revision: https://phab.mercurial-scm.org/D11620