avoid locking the journal in readonly repository

The test suite flagged that git-annex info in a readonly repository was
no longer working.

.git/annex/journal.lck: openFd: permission denied

This fixes it, however, in a case where .git/annex/reposize/ is
writable, but .git/annex/journal/ is not, there will still be a
permission denied error. The solution would just be to use consistent
permissions I suppose.

Database/RepoSize.hs

@@ -23,6 +23,7 @@ module Database.RepoSize (
 	getRepoSizeHandle,
 	openDb,
 	closeDb,
+	isOpenDb,
 	lockDbWhile,
 	getRepoSizes,
 	setRepoSizes,
@@ -126,6 +127,13 @@ openDb = lockDbWhile permerr $ do
 	-- efficiently.
 	permerr _e = mkhandle Nothing
 
+-- When the repository cannot be written to, openDb returns a
+-- RepoSizeHandle that is not actually open, all operations on it will do
+-- nothing.
+isOpenDb :: RepoSizeHandle -> Bool
+isOpenDb (RepoSizeHandle (Just _) _) = True
+isOpenDb (RepoSizeHandle Nothing _) = False
+
 closeDb :: RepoSizeHandle -> Annex ()
 closeDb (RepoSizeHandle (Just h) _) = liftIO $ H.closeDb h
 closeDb (RepoSizeHandle Nothing _) = noop