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.

Command/Smudge.hs

@@ -96,8 +96,8 @@ clean file = do
 				then B.length b `seq` return ()
 				else liftIO $ hClose stdin
 
-			-- Optimization when the file is already annexed
-			-- and is unmodified.
+			-- Optimization for the case when the file is already
+			-- annexed and is unmodified.
 			case oldkey of
 				Nothing -> doingest oldkey
 				Just ko -> ifM (isUnmodifiedCheap ko file)