dirstate-v2: add a new MTIME_SECOND_AMBIGUOUS flags
This flag will let us use more mtime value in the future. For now we have a
minimal handling of the flag at read time, but we will never put ourself in a
situation where we will needs to writes it.
See the flag documentation for details.
Differential Revision: https://phab.mercurial-scm.org/D11700