Sped up query commands that read the git-annex branch by around 5%

The only price paid is one additional MVar read per write to the journal.
Presumably writing a journal file dominiates over a MVar read time by
several orders of magnitude.

--batch does not get the speedup because then it needs to notice when
another process has made a change. Also made the assistant and other damon
modes bypass the optimisation, which would not help them anyway.

Benchmark.hs

@@ -28,7 +28,7 @@ mkGenerator cmds userinput = do
 	-- so skewing the runtime of the first action that will be
 	-- benchmarked.
 	Annex.Branch.commit "benchmarking"
-	Annex.Branch.update
+	_ <- Annex.Branch.update
 	l <- mapM parsesubcommand $ split [";"] userinput
 	return $ do
 		forM_ l $ \(cmd, seek, st) ->