Support nanosecond time resolution with stat() on macOS
Following on from #2606 (closed), it would be good to rework !2516 (merged) to not return 0
nanoseconds from _g_stat_mtim_nsec()
/_g_stat_atim_nsec()
/_g_stat_ctim_nsec()
, but instead return an actual nsec value, probably from the st_mtimespec.tv_nsec
field (etc.)
https://github.com/git/git/commit/c567383b1e3205c895b371d42a39fbdf131032ba seems to suggest this is possible. This suggests extra #define
s might be needed.