improve thirdpartypopulated support

May actually work now.

Note that, importKey now has to add the size to the key if it's supposed
to have size. Remote.Directory relied on the importer adding the size,
which is no longer done, so it was changed; it was the only one.
This way, importKey does not need to behave differently between regular
and thirdpartypopulated imports.

Remote/Directory.hs

@@ -371,9 +371,11 @@ guardSameContentIdentifiers cont old new
 	| otherwise = giveup "file content has changed"
 
 importKeyM :: RawFilePath -> ExportLocation -> ContentIdentifier -> ByteSize -> MeterUpdate -> Annex (Maybe Key)
-importKeyM dir loc cid _sz p = do
+importKeyM dir loc cid sz p = do
 	backend <- chooseBackend f
-	k <- fst <$> genKey ks p backend
+	unsizedk <- fst <$> genKey ks p backend
+	let k = alterKey unsizedk $ \kd -> kd
+		{ keySize = keySize kd <|> Just sz }
 	currcid <- liftIO $ mkContentIdentifier absf
 		=<< R.getFileStatus absf
 	guardSameContentIdentifiers (return (Just k)) cid currcid