force hash to finish with file before returning

Fixes a minor fd leak, never more than 1 in normal use,
which broke the test suite when I tried to write to
a file that was still open for a previous hashing.

Backend/Hash.hs

@@ -5,7 +5,7 @@
  - Licensed under the GNU GPL version 3 or higher.
  -}
 
-{-# LANGUAGE CPP #-}
+{-# LANGUAGE BangPatterns, CPP #-}
 
 module Backend.Hash (
 	backends,
@@ -171,7 +171,9 @@ hashFile hash file filesize = go hash
 	go (SHA3Hash hashsize) = use (sha3Hasher hashsize)
 	go (SkeinHash hashsize) = use (skeinHasher hashsize)
 	
-	use hasher = liftIO $ hasher <$> L.readFile file
+	use hasher = liftIO $ do
+		!h <- hasher <$> L.readFile file
+		return h
 	
 	usehasher hashsize = case shaHasher hashsize filesize of
 		Left sha -> use sha