add back git-annex branch read cache

The cache was removed way back in 2012,
commit 3417c55189

Then I forgot I had removed it! I remember clearly multiple times when I
thought, "this reads the same data twice, but the cache will avoid that
being very expensive".

The reason it was removed was it messed up the assistant noticing when
other processes made changes. That same kind of problem has recently
been addressed when adding the optimisation to avoid reading the journal
unnecessarily.

Indeed, enableInteractiveJournalAccess is run in just the
right places, so can just piggyback on it to know when it's not safe
to use the cache.

RemoteDaemon/Core.hs

@@ -166,7 +166,7 @@ genTransportHandle = do
 	let h = TransportHandle (LocalRepo g) annexstate
 	liftAnnex h $ do
 		Annex.setOutput QuietOutput
-		enableInteractiveJournalAccess
+		enableInteractiveBranchAccess
 	return h
 
 updateTransportHandle :: TransportHandle -> IO TransportHandle