convert configParser to Annex action and add passthrough option

Needed so Remote.External can query the external program for its
configs. When the external program does not support the query,
the passthrough option will make all input fields be available.

Remote/GitLFS.hs

@@ -60,7 +60,8 @@ remote = specialRemoteType $ RemoteType
 	-- and will call our gen on them.
 	, enumerate = const (return [])
 	, generate = gen
-	, configParser = [optionalStringParser urlField]
+	, configParser = mkRemoteConfigParser
+		[optionalStringParser urlField]
 	, setup = mySetup
 	, exportSupported = exportUnsupported
 	, importSupported = importUnsupported
@@ -133,7 +134,7 @@ mySetup _ mu _ c gc = do
 	u <- maybe (liftIO genUUID) return mu
 
 	(c', _encsetup) <- encryptionSetup c gc
-	pc <- either giveup return $ parseRemoteConfig c' (configParser remote)
+	pc <- either giveup return . parseRemoteConfig c' =<< configParser remote
 	case (isEncrypted pc, Git.GCrypt.urlPrefix `isPrefixOf` url) of
 		(False, False) -> noop
 		(True, True) -> Remote.GCrypt.setGcryptEncryption pc remotename