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.

Annex.hs

@@ -32,10 +32,6 @@ module Annex (
 	withCurrentState,
 ) where
 
-import "mtl" Control.Monad.Reader
-import Control.Monad.Catch
-import Control.Concurrent
-
 import Common
 import qualified Git
 import qualified Git.Config
@@ -62,11 +58,16 @@ import Types.LockPool
 import Types.MetaData
 import Types.DesktopNotify
 import Types.CleanupActions
-import qualified Data.Map as M
-import qualified Data.Set as S
 #ifdef WITH_QUVI
 import Utility.Quvi (QuviVersion)
 #endif
+import Utility.InodeCache
+
+import "mtl" Control.Monad.Reader
+import Control.Monad.Catch
+import Control.Concurrent
+import qualified Data.Map as M
+import qualified Data.Set as S
 
 {- git-annex's monad is a ReaderT around an AnnexState stored in a MVar.
  - This allows modifying the state in an exception-safe fashion.
@@ -120,7 +121,7 @@ data AnnexState = AnnexState
 	, fields :: M.Map String String
 	, modmeta :: [ModMeta]
 	, cleanup :: M.Map CleanupAction (Annex ())
-	, inodeschanged :: Maybe Bool
+	, sentinalstatus :: Maybe SentinalStatus
 	, useragent :: Maybe String
 	, errcounter :: Integer
 	, unusedkeys :: Maybe (S.Set Key)
@@ -165,7 +166,7 @@ newState c r = AnnexState
 	, fields = M.empty
 	, modmeta = []
 	, cleanup = M.empty
-	, inodeschanged = Nothing
+	, sentinalstatus = Nothing
 	, useragent = Nothing
 	, errcounter = 0
 	, unusedkeys = Nothing