add getFileSize, which can get the real size of a large file on Windows

Avoid using fileSize which maxes out at just 2 gb on Windows.
Instead, use hFileSize, which doesn't have a bounded size.
Fixes support for files > 2 gb on Windows.

Note that the InodeCache code only needs to compare a file size,
so it doesn't matter it the file size wraps. So it has been
left as-is. This was necessary both to avoid invalidating existing inode
caches, and because the code passed FileStatus around and would have become
more expensive if it called getFileSize.

This commit was sponsored by Christian Dietrich.

Backend/Hash.hs

@@ -68,8 +68,7 @@ hashNameE hash = hashName hash ++ "E"
 keyValue :: Hash -> KeySource -> Annex (Maybe Key)
 keyValue hash source = do
 	let file = contentLocation source
-	stat <- liftIO $ getFileStatus file
-	let filesize = fromIntegral $ fileSize stat
+	filesize <- liftIO $ getFileSize file
 	s <- hashFile hash file filesize
 	return $ Just $ stubKey
 		{ keyName = s
@@ -103,7 +102,7 @@ checkKeyChecksum hash key file = do
 	mstat <- liftIO $ catchMaybeIO $ getFileStatus file
 	case (mstat, fast) of
 		(Just stat, False) -> do
-			let filesize = fromIntegral $ fileSize stat
+			filesize <- liftIO $ getFileSize' file stat
 			showSideAction "checksum"
 			check <$> hashFile hash file filesize
 		_ -> return True