add a guard against using git annex add in direct mode repo

Currently, it deletes files when run in one, so until I get a chance to fix
it, block foot shooting.

Command.hs

@@ -18,6 +18,7 @@ module Command (
 	whenAnnexed,
 	ifAnnexed,
 	notBareRepo,
+	notDirect,
 	isBareRepo,
 	numCopies,
 	numCopiesCheck,
@@ -103,6 +104,12 @@ notBareRepo a = do
 		error "You cannot run this subcommand in a bare repository."
 	a
 
+notDirect :: Annex a -> Annex a
+notDirect a = ifM isDirect
+	( error "You cannot run this subcommand in a direct mode repository."
+	, a
+	)
+
 isBareRepo :: Annex Bool
 isBareRepo = fromRepo Git.repoIsLocalBare