git-annex config: guard against non-repo-global configs

git-annex config: Only allow configs be set that are ones git-annex
actually supports reading from repo-global config, to avoid confused users
trying to set other configs with this.

Types/Remote.hs

@@ -153,6 +153,9 @@ data RemoteA a = Remote
 	, remoteStateHandle :: RemoteStateHandle
 	}
 
+instance RemoteNameable (RemoteA a) where
+	getRemoteName = name
+
 instance Show (RemoteA a) where
 	show remote = "Remote { name =\"" ++ name remote ++ "\" }"