dirstate-v2: Add storage space for nanoseconds precision in file mtimes For now the sub-second component is always set to zero for tracked files and symlinks. (The mtime of directories for the `readdir`-skipping optimization is a different code path and already uses the full precision available.) This extra storage uses the space previously freed by replacing the 32-bit `mode` field by two bits in the existing `flags` field, so the overall size of nodes is unchanged. (This space had been left as padding for this purpose.) Also move things around in the node layout and documentation to have less duplication. Now that they have the same representation, directory mtime and file mtime are kept in the same field. (Only either one can exist for a given node.) Differential Revision: https://phab.mercurial-scm.org/D11655

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