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.

Types/GitConfig.hs

@@ -374,6 +374,7 @@ data RemoteGitConfig = RemoteGitConfig
 	, remoteAnnexAllowUnverifiedDownloads :: Bool
 	, remoteAnnexConfigUUID :: Maybe UUID
 	, remoteAnnexAllowEncryptedGitRepo :: Bool
+	, remoteUrl :: Maybe String
 
 	{- These settings are specific to particular types of remotes
 	 - including special remotes. -}
@@ -480,6 +481,12 @@ extractRemoteGitConfig r remotename = do
 		, remoteAnnexExternalType = notempty $ getmaybe "externaltype"
 		, remoteAnnexAllowEncryptedGitRepo = 
 			getbool "allow-encrypted-gitrepo" False
+		, remoteUrl = 
+			case Git.Config.getMaybe (remoteConfig remotename "url") r of
+				Just (ConfigValue b)
+					| B.null b -> Nothing
+					| otherwise -> Just (decodeBS b)
+				_ -> Nothing
 		}
   where
 	getbool k d = fromMaybe d $ getmaybebool k