dirstate: store mtimes with nanosecond precision in memory...
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
