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.hs

@@ -26,7 +26,6 @@ module Remote (
 	remoteTypes,
 	remoteList,
 	remoteList',
-	gitSyncableRemoteType,
 	remoteMap,
 	remoteMap',
 	uuidDescriptions,
@@ -61,6 +60,8 @@ module Remote (
 	claimingUrl,
 	claimingUrl',
 	isExportSupported,
+	gitSyncableRemote,
+	gitSyncableRemoteType,
 ) where
 
 import Data.Ord
@@ -445,3 +446,12 @@ claimingUrl' remotefilter url = do
 	fromMaybe web <$> firstM checkclaim (filter remotefilter rs)
   where
 	checkclaim = maybe (pure False) (`id` url) . claimUrl
+
+{- Is this a remote of a type we can sync with, or a special remote
+ - with an annex:: url configured? -}
+gitSyncableRemote :: Remote -> Bool
+gitSyncableRemote r
+	| gitSyncableRemoteType (remotetype r) = True
+	| otherwise = case remoteUrl (gitconfig r) of
+		Just u | "annex::" `isPrefixOf` u -> True
+		_ -> False