better name for fallback sync refs

Don't expose these as branches in refs/heads/. Instead hide them away in
refs/synced/ where only show-ref will find them.

Make unused only look at branches and tags, not these other things,
so it won't care if some stale sync ref used to use a file.

This means they don't need to be deleted, which could have
led to an incoming sync being missed.

Command/Unused.hs

@@ -251,7 +251,9 @@ withKeysReferencedInGit a = do
 	rs <- relevantrefs <$> showref
 	forM_ rs (withKeysReferencedInGitRef a)
 	where
-		showref = inRepo $ Git.Command.pipeRead [Param "show-ref"]
+		{- List heads and tags, but not other refs used in syncing. -}
+		showref = inRepo $ Git.Command.pipeRead
+			[Param "show-ref", Param "--heads", Param "--tags"]
 		relevantrefs = map (Git.Ref .  snd) .
 			nubBy uniqref .
 			filter ourbranches .