use bloom filter in second pass of sync --all --content

This is needed because when preferred content matches on files,
the second pass would otherwise want to drop all keys. Using a bloom filter
avoids this, and in the case of a false positive, a key will be left
undropped that preferred content would allow dropping. Chances of that
happening are a mere 1 in 1 million.

Types/Key.hs

@@ -132,8 +132,8 @@ instance Arbitrary Key where
 		<*> ((succ . abs <$>) <$> arbitrary) -- chunknum cannot be 0 or negative
 
 instance Hashable Key where
-	hashIO32 = hashIO32 . show
-	hashIO64 = hashIO64 . show
+	hashIO32 = hashIO32 . key2file
+	hashIO64 = hashIO64 . key2file
 
 prop_idempotent_key_encode :: Key -> Bool
 prop_idempotent_key_encode k = Just k == (file2key . key2file) k