implement fastDebug

Most of the changes here involve global option parsing: GlobalSetter
changed so it can both run an Annex action to set state, but can also
change the AnnexRead value, which is immutable once the Annex monad is
running.

That allowed a debugselector value to be added to AnnexRead, seeded
from the git config. The --debugfilter option's GlobalSetter then updates
the AnnexRead.

This improved GlobalSetter can later be used to move more stuff to
AnnexRead. Things that don't involve a git config will be easier to
move, and probably a *lot* of things can be moved eventually.

fastDebug, while implemented, is not used anywhere yet. But it should be
fast..

CmdLine.hs

@@ -53,14 +53,14 @@ dispatch' subcommandname args fuzzy cmds allargs allcmds fields getgitrepo progn
   where
 	go (Right g) = do
 		g' <- Git.Config.read g
-		(cmd, seek, globalconfig) <- parsewith False cmdparser
+		(cmd, seek, globalsetter) <- parsewith False cmdparser
 			(\a -> a (Just g'))
 			O.handleParseResult
-		state <- Annex.new g'
+		state <- applyAnnexReadSetter globalsetter <$> Annex.new g'
 		Annex.eval state $ do
 			checkEnvironment
 			forM_ fields $ uncurry Annex.setField
-			prepRunCommand cmd globalconfig
+			prepRunCommand cmd globalsetter
 			startup
 			performCommandAction cmd seek $
 				shutdown $ cmdnocommit cmd
@@ -141,11 +141,14 @@ subCmdName argv = (name, args)
 		| "-" `isPrefixOf` a = findname as (a:c)
 		| otherwise = (Just a, reverse c ++ as)
 
+-- | Note that the GlobalSetter must have already had its annexReadSetter
+-- applied before entering the Annex monad; that cannot be changed while
+-- running in the Annex monad.
 prepRunCommand :: Command -> GlobalSetter -> Annex ()
-prepRunCommand cmd globalconfig = do
+prepRunCommand cmd globalsetter = do
 	when (cmdnomessages cmd) $
 		Annex.setOutput QuietOutput
-	getParsed globalconfig
+	annexStateSetter globalsetter
 	whenM (annexDebug <$> Annex.getGitConfig) $
 		enableDebugOutput