Tahoe: Avoid verifying hash after download, since tahoe does sufficient verification itself

See my comment in the next commit for some details about why
Verified needs a hash with preimage resistance. As far as tahoe goes,
it's fully cryptographically secure.

I think that bup could also return Verified. However, the Retriever
interface does not currenly support that.

Remote/BitTorrent.hs

@@ -99,6 +99,9 @@ gen r _ rc gc rs = do
 downloadKey :: Key -> AssociatedFile -> FilePath -> MeterUpdate -> Annex Verification
 downloadKey key _file dest p = do
 	get . map (torrentUrlNum . fst . getDownloader) =<< getBitTorrentUrls key
+	-- While bittorrent verifies the hash in the torrent file,
+	-- the torrent file itself is downloaded without verification,
+	-- so the overall download is not verified.
 	return UnVerified
   where
 	get [] = giveup "could not download torrent"

Remote/Tahoe.hs

@@ -147,7 +147,9 @@ store rs hdl k _f _p = sendAnnex k noop $ \src ->
 retrieve :: RemoteStateHandle -> TahoeHandle -> Key -> AssociatedFile -> FilePath -> MeterUpdate -> Annex Verification
 retrieve rs hdl k _f d _p = do
 	go =<< getCapability rs k
-	return UnVerified
+	-- Tahoe verifies the content it retrieves using cryptographically
+	-- secure methods.
+	return Verified
   where
 	go Nothing = giveup "tahoe capability is not known"
 	go (Just cap) = unlessM (liftIO $ requestTahoe hdl "get" [Param cap, File d]) $