get rid of racy addLink

The remaining callers all did not rely on it checking gitignore, so were
easy to convert.

They were susceptable to the same overwrite race as add and fix,
although less likely to have it and a narrower window than add's race.

Command.Rekey in passing got an unncessary call to removeFile deleted.
addSymlink handles deleting any existing worktree file.

Command/AddUnused.hs

@@ -30,9 +30,7 @@ start = startUnused "addunused" perform
 perform :: Key -> CommandPerform
 perform key = next $ do
 	logStatus key InfoPresent
-	-- Ignore the usual git ignores because the user has explictly
-	-- asked to add these files.
-	addLink (CheckGitIgnore False) file key Nothing
+	addSymlink file key Nothing
 	return True
   where
 	file = "unused." <> keyFile key