add, import, assistant: Better preserve the mtime of symlinks, when when adding content that gets deduplicated.

Note that this turned out to remove a syscall, not add any expense.
Otherwise, I would not have done it.

Command/ReKey.hs

@@ -66,6 +66,6 @@ cleanup file oldkey newkey = do
 
 	-- Update symlink to use the new key.
 	liftIO $ removeFile file
-	Command.Add.addLink file newkey True
+	Command.Add.addLink file newkey Nothing
 	logStatus newkey InfoPresent
 	return True