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.

Logs/Web.hs

@@ -90,7 +90,7 @@ knownUrls = do
 	{- Ensure the git-annex branch's index file is up-to-date and
 	 - any journaled changes are reflected in it, since we're going
 	 - to query its index directly. -}
-	Annex.Branch.update
+	_ <- Annex.Branch.update
 	Annex.Branch.commit =<< Annex.Branch.commitMessage
 	Annex.Branch.withIndex $ do
 		top <- fromRepo Git.repoPath