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/AddUnused.hs

@@ -29,7 +29,7 @@ start = startUnused "addunused" perform
 perform :: Key -> CommandPerform
 perform key = next $ do
 	logStatus key InfoPresent
-	Command.Add.addLink file key False
+	Command.Add.addLink file key Nothing
 	return True
   where
 	file = "unused." ++ key2file key