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.

Annex/Content.hs

@@ -766,6 +766,10 @@ isUnmodified key f = go =<< geti
 
 {- Cheap check if a file contains the unmodified content of the key,
  - only checking the InodeCache of the key.
+ -
+ - Note that, on systems not supporting high-resolution mtimes,
+ - this may report a false positive when repeated edits are made to a file
+ - within a small time window (eg 1 second).
  -}
 isUnmodifiedCheap :: Key -> FilePath -> Annex Bool
 isUnmodifiedCheap key f = maybe (return False) (isUnmodifiedCheap' key)