disable journal read optimisation when alwayscommit=false

The journal read optimisation in aeca7c220 later got fixed in eedd73b84
to stage and commit any files that were left in the journal by a
previous git-annex run. That's necessary for the optimisation to work
correctly. But it also meant that alwayscommit=false started committing
the previous git-annex processes journalled changes, which defeated the
purpose of the config setting entirely.

So, disable the optimisation when alwayscommit=false, leaving the
files in the journal and not committing them. See my comments on the bug
report for why this seemed the best approach.

Also fixes a problem when annex.merge-annex-branches=false and there
are changes in the journal. That config indirectly prevents committing
the journal. (Which seems a bit odd given its name, but it always has..)
So, when there were changes in the journal, perhaps left there due to
alwayscommit=false being set before, the optimisation would prevent
git-annex from reading the journal files, and it would operate with out
of date information.

Annex/BranchState.hs

@@ -28,26 +28,24 @@ checkIndexOnce a = unlessM (indexChecked <$> getState) $ do
 	changeState $ \s -> s { indexChecked = True }
 
 {- Runs an action to update the branch, if it's not been updated before
- - in this run of git-annex. -}
-runUpdateOnce :: Annex () -> Annex BranchState
+ - in this run of git-annex. 
+ -
+ - The action should return True if anything that was in the journal
+ - before got staged (or if the journal was empty). That lets an opmisation
+ - be done: The journal then does not need to be checked going forward,
+ - until new information gets written to it.
+ -}
+runUpdateOnce :: Annex Bool -> Annex BranchState
 runUpdateOnce a = do
 	st <- getState
 	if branchUpdated st
 		then return st
 		else do
-			a
+			journalstaged <- a
 			let stf = \st' -> st'
 				{ branchUpdated = True
-				-- The update staged anything that was
-				-- journalled before, so the journal
-				-- does not need to be checked going
-				-- forward, unless new information
-				-- gets written to it, or unless
-				-- this run of git-annex needs to notice
-				-- changes journalled by other processes
-				-- while it's running.
-				, journalIgnorable = not $
-					journalNeverIgnorable st'
+				, journalIgnorable = journalstaged 
+					&& not (journalNeverIgnorable st')
 				}
 			changeState stf
 			return (stf st)