finish fixing windows timezone madness

Rather than calculating the TSDelta once, and caching it, this now
reads the inode sential file's InodeCache file once, and then each time a
new InodeCache is generated, looks at the sentinal file to get the current
delta.

This way, if the time zone changes while git-annex is running, it will
adapt.

This adds some inneffiency, but only on Windows, and only 1 stat per new
file added. The worst innefficiency is that `git annex status` and
`git annex sync` will now (on Windows) stat the inode sentinal file once per
file in the repo.

It would be more efficient to use getCurrentTimeZone, rather than needing
to stat the sentinal file. This should be easy to do, once the time
package gets my bugfix patch.

This commit was sponsored by Jürgen Lüters.

Command/Add.hs

@@ -155,7 +155,7 @@ ingest (Just source) = withTSDelta $ \delta -> do
 	backend <- chooseBackend $ keyFilename source
 	k <- genKey source backend
 	ms <- liftIO $ catchMaybeIO $ getFileStatus $ contentLocation source
-	let mcache = toInodeCache delta =<< ms
+	mcache <- maybe (pure Nothing) (liftIO . toInodeCache delta) ms
 	case (mcache, inodeCache source) of
 		(_, Nothing) -> go k mcache ms
 		(Just newc, Just c) | compareStrong c newc -> go k mcache ms