The file index is limited to 1s resolution, which means that scripted use of darcs needs to use --
ignore-times to avoid it incorrectly reporting that files haven't changed.
There are at options for getting higher resolution timestamps from at least some filesystems: we
should use them if possible.
- See http://hackage.haskell.org/trac/ghc/ticket/7473
- POSIX.1-2008 specifies st_atim.tv_nsec
- Win32 API needs to be done separately
- Need to bump index version and widen the timestamp