make WorkerStage an open type

Rather than limiting it to PerformStage and CleanupStage, this opens it
up so any number of stages can be added as needed by commands.

Each concurrent command has a set of stages that it uses, and only
transitions between those can block waiting for a free slot in the
worker pool. Calling enteringStage for some other stage does not block,
and has very little overhead.

Note that while before the Annex state was duplicated on the first call
to commandAction, this now happens earlier, in startConcurrency.
That means that seek stage actions should that use startConcurrency
and then modify Annex state won't modify the state of worker threads
they then start. I audited all of them, and only Command.Seek
did so; prepMerge changes the working directory and so has to come
before startConcurrency.

Also, the remote list is built before duplicating the state, which means
that it gets built earlier now than it used to. This would only have an
effect of making commands that end up not needing to perform any actions
unncessary build the remote list (only when they're run with concurrency
enable), but that's a minor overhead compared to commands seeking
through the work tree and determining they don't need to do anything.

Command/Add.hs

@@ -50,7 +50,7 @@ optParser desc = AddOptions
 		)
 
 seek :: AddOptions -> CommandSeek
-seek o = allowConcurrentOutput $ do
+seek o = startConcurrency commandStages $ do
 	matcher <- largeFilesMatcher
 	let gofile file = ifM (checkFileMatcher matcher file <||> Annex.getState Annex.force)
 		( start file

Command/AddUrl.hs

@@ -93,7 +93,7 @@ parseDownloadOptions withfileoption = DownloadOptions
 		else pure Nothing
 
 seek :: AddUrlOptions -> CommandSeek
-seek o = allowConcurrentOutput $ do
+seek o = startConcurrency commandStages $ do
 	forM_ (addUrls o) (\u -> go (o, u))
 	case batchOption o of
 		Batch fmt -> batchInput fmt (parseBatchInput o) go

Command/Copy.hs

@@ -44,7 +44,7 @@ instance DeferredParseClass CopyOptions where
 		<*> pure (batchOption v)
 
 seek :: CopyOptions -> CommandSeek
-seek o = allowConcurrentOutput $ do
+seek o = startConcurrency commandStages $ do
 	let go = whenAnnexed $ start o
 	case batchOption o of
 		Batch fmt -> batchFilesMatching fmt go

Command/Drop.hs

@@ -52,7 +52,7 @@ parseDropFromOption = parseRemoteOption <$> strOption
 	)
 
 seek :: DropOptions -> CommandSeek
-seek o = allowConcurrentOutput $
+seek o = startConcurrency commandStages $
 	case batchOption o of
 		Batch fmt -> batchFilesMatching fmt go
 		NoBatch -> withKeyOptions (keyOptions o) (autoMode o)

Command/Fsck.hs

@@ -88,7 +88,7 @@ optParser desc = FsckOptions
 			))
 
 seek :: FsckOptions -> CommandSeek
-seek o = allowConcurrentOutput $ do
+seek o = startConcurrency commandStages $ do
 	from <- maybe (pure Nothing) (Just <$$> getParsed) (fsckFromOption o)
 	u <- maybe getUUID (pure . Remote.uuid) from
 	checkDeadRepo u

Command/Get.hs

@@ -38,7 +38,7 @@ optParser desc = GetOptions
 	<*> parseBatchOption
 
 seek :: GetOptions -> CommandSeek
-seek o = allowConcurrentOutput $ do
+seek o = startConcurrency commandStages $ do
 	from <- maybe (pure Nothing) (Just <$$> getParsed) (getFrom o)
 	let go = whenAnnexed $ start o from
 	case batchOption o of

Command/Import.hs

@@ -96,7 +96,7 @@ duplicateModeParser =
 		)
 
 seek :: ImportOptions -> CommandSeek
-seek o@(LocalImportOptions {}) = allowConcurrentOutput $ do
+seek o@(LocalImportOptions {}) = startConcurrency commandStages $ do
 	repopath <- liftIO . absPath =<< fromRepo Git.repoPath
 	inrepops <- liftIO $ filter (dirContains repopath) <$> mapM absPath (importFiles o)
 	unless (null inrepops) $ do
@@ -104,7 +104,7 @@ seek o@(LocalImportOptions {}) = allowConcurrentOutput $ do
 	largematcher <- largeFilesMatcher
 	(commandAction . startLocal largematcher (duplicateMode o))
 		`withPathContents` importFiles o
-seek o@(RemoteImportOptions {}) = allowConcurrentOutput $ do
+seek o@(RemoteImportOptions {}) = startConcurrency commandStages $ do
 	r <- getParsed (importFromRemote o)
 	unlessM (Remote.isImportSupported r) $
 		giveup "That remote does not support imports."

Command/Mirror.hs

@@ -41,7 +41,7 @@ instance DeferredParseClass MirrorOptions where
 		<*> pure (keyOptions v)
 
 seek :: MirrorOptions -> CommandSeek
-seek o = allowConcurrentOutput $ 
+seek o = startConcurrency commandStages $ 
 	withKeyOptions (keyOptions o) False
 		(commandAction . startKey o (AssociatedFile Nothing))
 		(withFilesInGit (commandAction . (whenAnnexed $ start o)))

Command/Move.hs

@@ -54,7 +54,7 @@ data RemoveWhen = RemoveSafe | RemoveNever
 	deriving (Show, Eq)
 
 seek :: MoveOptions -> CommandSeek
-seek o = allowConcurrentOutput $ do
+seek o = startConcurrency commandStages $ do
 	let go = whenAnnexed $ start (fromToOptions o) (removeWhen o)
 	case batchOption o of
 		Batch fmt -> batchFilesMatching fmt go

Command/Sync.hs

@@ -162,9 +162,12 @@ instance DeferredParseClass SyncOptions where
 		<*> pure (resolveMergeOverride v)
 
 seek :: SyncOptions -> CommandSeek
-seek o = allowConcurrentOutput $ do
+seek o = do
 	prepMerge
-
+	startConcurrency commandStages (seek' o)
+	
+seek' :: SyncOptions -> CommandSeek
+seek' o = do
 	let withbranch a = a =<< getCurrentBranch
 
 	remotes <- syncRemotes (syncWith o)