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
[build-system]
requires = ["setuptools", "wheel"]
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true