ensure tmp file is writable, so rsync can resume

It's possible that rsync finishes transferring a file and sets its mode,
but the file transfer to the annex then fails. When resuming, rsync
would then not be able to write to the tmp file.

Content.hs

@@ -103,6 +103,8 @@ getViaTmp key action = do
 			checkDiskSpace' (fromIntegral $ fileSize stat) key
 		else checkDiskSpace key
 
+	when e $ liftIO $ allowWrite tmp
+
 	getViaTmpUnchecked key action
 
 {- Like getViaTmp, but does not check that there is enough disk space