display "Recording state in git..." when staging the journal

A bit tricky to avoid printing it twice in a row when there are queued git
commands to run and journal to stage.

Added a generic way to run an action that may output multiple side
messages, with only the first displayed.

Annex/Branch.hs

@@ -330,6 +330,7 @@ setCommitted = void $ do
 {- Stages the journal into the index. -}
 stageJournal :: Annex ()
 stageJournal = do
+	showStoringStateAction
 	fs <- getJournalFiles
 	g <- gitRepo
 	withIndex $ liftIO $ do