limit git annex adjust to v6 mode

doesn't work in v5

Annex/Version.hs

@@ -52,6 +52,9 @@ versionSupportsUnlockedPointers = go <$> getVersion
 	go (Just "6") = True
 	go _ = False
 
+versionSupportsAdjustedBranch :: Annex Bool
+versionSupportsAdjustedBranch = versionSupportsUnlockedPointers
+
 setVersion :: Version -> Annex ()
 setVersion = setConfig versionField
 

Command/Adjust.hs

@@ -9,6 +9,7 @@ module Command.Adjust where
 
 import Command
 import Annex.AdjustedBranch
+import Annex.Version
 
 cmd :: Command
 cmd = notBareRepo $ notDirect $
@@ -33,5 +34,7 @@ seek = commandAction . start
 
 start :: Adjustment -> CommandStart
 start adj = do
+	unlessM versionSupportsAdjustedBranch $
+		error "Adjusted branches are only supported in v6 or newer repositories."
 	enterAdjustedBranch adj
 	next $ next $ return True