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.

Command/Forget.hs

@@ -45,7 +45,7 @@ perform :: Transitions -> Bool -> CommandPerform
 perform ts True = do
 	recordTransitions Branch.change ts
 	-- get branch committed before contining with the transition
-	Branch.update
+	_ <- Branch.update
 	void $ Branch.performTransitions ts True []
 	next $ return True
 perform _ False = do