high-res mtimes

Cache high-resolution mtimes for improved detection of modified files in v7
(and direct mode).

Including on Windows.

With back-compat support so old low-res mtimes won't break anything, and
so the new information also won't break old versions of git-annex.

Utility/QuickCheck.hs

@@ -31,7 +31,7 @@ instance (Arbitrary v, Ord v) => Arbitrary (S.Set v) where
 	arbitrary = S.fromList <$> arbitrary
 #endif
 
-{- Times before the epoch are excluded. -}
+{- Times before the epoch are excluded, and no fraction is included. -}
 instance Arbitrary POSIXTime where
 	arbitrary = fromInteger <$> nonNegative arbitrarySizedIntegral