fix for Windows file timestamp timezone madness

On Windows, changing the time zone causes the apparent mtime of files to
change. This confuses git-annex, which natually thinks this means the files
have actually been modified (since THAT'S WHAT A MTIME IS FOR, BILL <sheesh>).

Work around this stupidity, by using the inode sentinal file to detect if
the timezone has changed, and calculate a TSDelta, which will be applied
when generating InodeCaches.

This should add no overhead at all on unix. Indeed, I sped up a few
things slightly in the refactoring.

Seems to basically work! But it has a big known problem:
If the timezone changes while the assistant (or a long-running command)
runs, it won't notice, since it only checks the inode cache once, and
so will use the old delta for all new inode caches it generates for new
files it's added. Which will result in them seeming changed the next time
it runs.

This commit was sponsored by Vincent Demeester.

Command/Add.hs

@@ -102,7 +102,7 @@ lockDown = either (\e -> showErr e >> return Nothing) (return . Just) <=< lockDo
 
 lockDown' :: FilePath -> Annex (Either IOException KeySource)
 lockDown' file = ifM crippledFileSystem
-	( liftIO $ tryIO nohardlink
+	( withTSDelta $ liftIO . tryIO . nohardlink
 	, tryAnnexIO $ do
 		tmp <- fromRepo gitAnnexTmpMiscDir
 		createAnnexDirectory tmp
@@ -122,22 +122,22 @@ lockDown' file = ifM crippledFileSystem
   	go tmp = do
 		unlessM isDirect $
 			freezeContent file
-		liftIO $ do
+		withTSDelta $ \delta -> liftIO $ do
 			(tmpfile, h) <- openTempFile tmp $
 				relatedTemplate $ takeFileName file
 			hClose h
 			nukeFile tmpfile
-			withhardlink tmpfile `catchIO` const nohardlink
-  	nohardlink = do
-		cache <- genInodeCache file
+			withhardlink delta tmpfile `catchIO` const (nohardlink delta)
+  	nohardlink delta = do
+		cache <- genInodeCache file delta
 		return KeySource
 			{ keyFilename = file
 			, contentLocation = file
 			, inodeCache = cache
 			}
-	withhardlink tmpfile = do
+	withhardlink delta tmpfile = do
 		createLink file tmpfile
-		cache <- genInodeCache tmpfile
+		cache <- genInodeCache tmpfile delta
 		return KeySource
 			{ keyFilename = file
 			, contentLocation = tmpfile
@@ -151,11 +151,11 @@ lockDown' file = ifM crippledFileSystem
  -}
 ingest :: Maybe KeySource -> Annex (Maybe Key, Maybe InodeCache)
 ingest Nothing = return (Nothing, Nothing)
-ingest (Just source) = do
+ingest (Just source) = withTSDelta $ \delta -> do
 	backend <- chooseBackend $ keyFilename source
 	k <- genKey source backend
 	ms <- liftIO $ catchMaybeIO $ getFileStatus $ contentLocation source
-	let mcache = toInodeCache =<< ms
+	let mcache = toInodeCache delta =<< ms
 	case (mcache, inodeCache source) of
 		(_, Nothing) -> go k mcache ms
 		(Just newc, Just c) | compareStrong c newc -> go k mcache ms