added git-annex extendcluster

This works, but updatecluster does not work yet in multi-gateway
clusters, nor do gateways relay to other gateways.

Command/ExtendCluster.hs

@@ -0,0 +1,50 @@
+{- git-annex command
+ -
+ - Copyright 2024 Joey Hess <id@joeyh.name>
+ -
+ - Licensed under the GNU AGPL version 3 or higher.
+ -}
+
+{-# LANGUAGE OverloadedStrings #-}
+
+module Command.ExtendCluster where
+
+import Command
+import qualified Annex
+import Types.Cluster
+import Config
+import qualified Remote
+
+import qualified Data.Map as M
+
+cmd :: Command
+cmd = command "extendcluster" SectionSetup "add an gateway to a cluster"
+	(paramPair paramRemote paramName) (withParams seek)
+
+seek :: CmdParams -> CommandSeek
+seek (remotename:clustername:[]) = Remote.byName (Just clusterremotename) >>= \case
+	Just clusterremote -> 
+		case mkClusterUUID (Remote.uuid clusterremote) of
+			Just cu -> commandAction $ start cu clustername
+			Nothing -> giveup $ clusterremotename 
+				++ " is not a cluster remote."
+	Nothing -> giveup $ "Expected to find a cluster remote named " 
+		++ clusterremotename
+		++ " that is accessed via " ++ remotename
+		++ ", but there is no such remote. Perhaps you need to"
+		++ "git fetch from " ++ remotename 
+		++ ", or git-annex updatecluster needs to be run there?"
+  where
+	clusterremotename = remotename ++ "-" ++ clustername
+seek _ = giveup "Expected two parameters, gateway and clustername."
+
+start :: ClusterUUID -> String -> CommandStart
+start cu clustername = starting "extendcluster" ai si $ do
+	myclusters <- annexClusters <$> Annex.getGitConfig
+	unless (M.member clustername myclusters) $ do
+		setConfig (annexConfig ("cluster." <> encodeBS clustername))
+			(fromUUID (fromClusterUUID cu))
+	next $ return True
+  where
+	ai = ActionItemOther (Just (UnquotedString clustername))
+	si = SeekInput [clustername]