make CommandStart return a StartMessage

The goal is to be able to run CommandStart in the main thread when -J is
used, rather than unncessarily passing it off to a worker thread, which
incurs overhead that is signficant when the CommandStart is going to
quickly decide to stop.

To do that, the message it displays needs to be displayed in the worker
thread, after the CommandStart has run.

Also, the change will mean that CommandStart will no longer necessarily
run with the same Annex state as CommandPerform. While its docs already
said it should avoid modifying Annex state, I audited all the
CommandStart code as part of the conversion. (Note that CommandSeek
already sometimes runs with a different Annex state, and that has not been
a source of any problems, so I am not too worried that this change will
lead to breakage going forward.)

The only modification of Annex state I found was it calling
allowMessages in some Commands that default to noMessages. Dealt with
that by adding a startCustomOutput and a startingUsualMessages.
This lets a command start with noMessages and then select the output it
wants for each CommandStart.

One bit of breakage: onlyActionOn has been removed from commands that used it.
The plan is that, since a StartMessage contains an ActionItem,
when a Key can be extracted from that, the parallel job runner can
run onlyActionOn' automatically. Then commands won't need to worry about
this detail. Future work.

Otherwise, this was a fairly straightforward process of making each
CommandStart compile again. Hopefully other behavior changes were mostly
avoided.

In a few cases, a command had a CommandStart that called a CommandPerform
that then called showStart multiple times. I have collapsed those
down to a single start action. The main command to perhaps suffer from it
is Command.Direct, which used to show a start for each file, and no
longer does.

Another minor behavior change is that some commands used showStart
before, but had an associated file and a Key available, so were changed
to ShowStart with an ActionItemAssociatedFile. That will not change the
normal output or behavior, but --json output will now include the key.
This should not break it for anyone using a real json parser.

Command.hs

@@ -22,14 +22,12 @@ import CmdLine.GlobalSetter as ReExported
 import CmdLine.GitAnnex.Options as ReExported
 import CmdLine.Batch as ReExported
 import Options.Applicative as ReExported hiding (command)
-import qualified Annex
 import qualified Git
 import Annex.Init
 import Config
 import Utility.Daemon
 import Types.Transfer
 import Types.ActionItem
-import Types.Messages
 
 {- Generates a normal Command -}
 command :: String -> CommandSection -> String -> CmdParamsDesc -> (CmdParamsDesc -> CommandParser) -> Command
@@ -61,19 +59,11 @@ noCommit c = c { cmdnocommit = True }
  - starting or stopping processing a file or other item. Unless --json mode
  - is enabled, this also enables quiet output mode, so only things
  - explicitly output by the command are shown and not progress messages
- - etc. -}
+ - etc.
+ -}
 noMessages :: Command -> Command
 noMessages c = c { cmdnomessages = True }
 
-{- Undoes noMessages -}
-allowMessages :: Annex ()
-allowMessages = do
-	outputType <$> Annex.getState Annex.output >>= \case
-		QuietOutput -> Annex.setOutput NormalOutput
-		_ -> noop
-	Annex.changeState $ \s -> s
-		{ Annex.output = (Annex.output s) { implicitMessages = True } }
-
 {- Adds a fallback action to a command, that will be run if it's used
  - outside a git repository. -}
 noRepo :: (String -> Parser (IO ())) -> Command -> Command
@@ -83,11 +73,25 @@ noRepo a c = c { cmdnorepo = Just (a (cmdparamdesc c)) }
 withGlobalOptions :: [[GlobalOption]] -> Command -> Command
 withGlobalOptions os c = c { cmdglobaloptions = cmdglobaloptions c ++ concat os }
 
-{- For start and perform stages to indicate what step to run next. -}
+{- For start stage to indicate what will be done. -}
+starting:: MkActionItem t => String -> t -> CommandPerform -> CommandStart
+starting msg t a = next (StartMessage msg (mkActionItem t), a)
+
+{- Use when noMessages was used but the command is going to output
+ - usual messages after all. -}
+startingUsualMessages :: MkActionItem t => String -> t -> CommandPerform -> CommandStart
+startingUsualMessages msg t a = next (StartUsualMessages msg (mkActionItem t), a)
+
+{- For commands that do not display usual start or end messages, 
+ - but have some other custom output. -}
+startingCustomOutput :: CommandPerform -> CommandStart
+startingCustomOutput a = next (CustomOutput, a)
+
+{- For perform stage to indicate what step to run next. -}
 next :: a -> Annex (Maybe a)
 next a = return $ Just a
 
-{- Or to indicate nothing needs to be done. -}
+{- For start and perform stage to indicate nothing needs to be done. -}
 stop :: Annex (Maybe a)
 stop = return Nothing