rhg: Set second_ambiguous as needed in post-status fixup This fixes an intermittent bug that manifested only in test-revert.t, and unfortunately not on CI. On a fast enough machine we could have: 1. A file is modified 2. `rhg status` writes an updated dirstate-v1 3. The same file is modified again … all within the same integer second. Because the dirstate-v1 file format does not store sub-second precision, step 2 must write the file’s mtime as "unknown" because of the possibility of step 3. However, most of the code now handles timestamps with nanosecond precision in order to take advantage of it in dirstate-v2. `second_ambiguous` must be set for timestamps that become ambiguous if sub-second precision is dropped (such as through serialization in dirstate-v1 format). Differential Revision: https://phab.mercurial-scm.org/D11889