sync with special remotes with an annex:: url

Check explicitly for an annex:: url, not just any url. While no built-in
special remotes set an url, except ones that can be synced with, it
seems possible that some external special remote sets an url for its own
use, but did not expect it to be used by git-annex sync et al.

The assistant also syncs with them.

Remote/List.hs

@@ -109,7 +109,11 @@ updateRemote remote = do
 			Remote.Git.configRead False r
 		| otherwise = return r
 
-{- Checks if a remote is syncable using git. -}
+{- Types of remotes that are always syncable using git.
+ -
+ - This does not include special remotes that may or may not have an
+ - annex:: url that allows using git-remote-annex with them.
+ -}
 gitSyncableRemoteType :: RemoteType -> Bool
 gitSyncableRemoteType t = t `elem`
 	[ Remote.Git.remote