dirstate: store mtimes with nanosecond precision in memory Keep integer seconds since the Unix epoch, together with integer nanoseconds in the `0 <= n < 1e9` range. For now, nanoseconds are still always zero. This commit is about data structure changes. Differential Revision: https://phab.mercurial-scm.org/D11684

rust: remove now-unused DirstateEntry::clear_ambiguous_mtime method Differential Revision: https://phab.mercurial-scm.org/D11696

dirstate: align Rust function name to `need_delay` The rest of the code use this name. It is not a great name, but it is better to stay consistent. Differential Revision: https://phab.mercurial-scm.org/D11699

dirstate: rename a `very_likely_equal` method to `likely_equal` No need to oversell it. Differential Revision: https://phab.mercurial-scm.org/D11691

backout: backed out changeset 2f2107c01dee After discussion on the original phab patch, we've decided to wait until Python 2 support is dropped to do this since the behavior differs for relatively obscure reasons. Differential Revision: https://phab.mercurial-scm.org/D11692

backout: backed out changeset 6edc8800dbc3 Same as the previous changeset. Differential Revision: https://phab.mercurial-scm.org/D11693

dirstate-v2: preserve the fallback values on disk When the fallback values are set, they are now read and written to disk. See format documentation for details. Differential Revision: https://phab.mercurial-scm.org/D11688