stream through proxy when using fileRetriever

The problem was that when the proxy requests a key be retrieved to its
own temp file, fileRetriever was retriving it to the key's temp
location, and then moving it at the end, which broke streaming.

So, plumb through the path where the key is being retrieved to.

Remote/Helper/Special.hs

@@ -42,6 +42,7 @@ import Types.StoreRetrieve
 import Types.Remote
 import Annex.Verify
 import Annex.UUID
+import Annex.Perms
 import Config
 import Config.Cost
 import Utility.Metered
@@ -106,10 +107,10 @@ byteStorer a k c m = withBytes c $ \b -> a k b m
 -- A Retriever that generates a lazy ByteString containing the Key's
 -- content, and passes it to a callback action which will fully consume it
 -- before returning.
-byteRetriever :: (Key -> (L.ByteString -> Annex a) -> Annex a) -> Key -> MeterUpdate -> Maybe IncrementalVerifier -> (ContentSource -> Annex a) -> Annex a
-byteRetriever a k _m _miv callback = a k (callback . ByteContent)
+byteRetriever :: (Key -> (L.ByteString -> Annex a) -> Annex a) -> Key -> MeterUpdate -> RawFilePath -> Maybe IncrementalVerifier -> (ContentSource -> Annex a) -> Annex a
+byteRetriever a k _m _dest _miv callback = a k (callback . ByteContent)
 
--- A Retriever that writes the content of a Key to a provided file.
+-- A Retriever that writes the content of a Key to a file.
 -- The action is responsible for updating the progress meter as it 
 -- retrieves data. The incremental verifier is updated in the background as
 -- the action writes to the file, but may not be updated with the entire
@@ -119,15 +120,15 @@ fileRetriever a = fileRetriever' $ \f k m miv ->
 	let retrieve = a f k m
 	in tailVerify miv f retrieve
 
-{- A Retriever that writes the content of a Key to a provided file.
+{- A Retriever that writes the content of a Key to a file.
  - The action is responsible for updating the progress meter and the 
  - incremental verifier as it retrieves data.
  -}
 fileRetriever' :: (RawFilePath -> Key -> MeterUpdate -> Maybe IncrementalVerifier -> Annex ()) -> Retriever
-fileRetriever' a k m miv callback = do
-	f <- prepTmp k
-	a f k m miv
-	pruneTmpWorkDirBefore f (callback . FileContent . fromRawFilePath)
+fileRetriever' a k m dest miv callback = do
+	createAnnexDirectory (parentDir dest)
+	a dest k m miv
+	pruneTmpWorkDirBefore dest (callback . FileContent . fromRawFilePath)
 
 {- The base Remote that is provided to specialRemote needs to have
  - storeKey, retrieveKeyFile, removeKey, and checkPresent methods,