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