type based git config handling

Now there's a Config type, that's extracted from the git config at startup.
Note that laziness means that individual config values are only looked up
and parsed on demand, and so we get implicit memoization for all of them.
So this is not only prettier and more type safe, it optimises several
places that didn't have explicit memoization before. As well as getting rid
of the ugly explicit memoization code.

Not yet done for annex.<remote>.* configuration settings.

Backend.hs

@@ -18,7 +18,6 @@ module Backend (
 import System.Posix.Files
 
 import Common.Annex
-import Config
 import qualified Annex
 import Annex.CheckAttr
 import Types.Key
@@ -39,17 +38,18 @@ orderedList = do
 	l <- Annex.getState Annex.backends -- list is cached here
 	if not $ null l
 		then return l
-		else handle =<< Annex.getState Annex.forcebackend
+		else do
+			f <- Annex.getState Annex.forcebackend
+			case f of
+				Just name | not (null name) ->
+					return [lookupBackendName name]
+				_ -> do
+					l' <- gen . annexBackends <$> Annex.getConfig
+					Annex.changeState $ \s -> s { Annex.backends = l' }
+					return l'
   where
-	handle Nothing = standard
-	handle (Just "") = standard
-	handle (Just name) = do
-		l' <- (lookupBackendName name :) <$> standard
-		Annex.changeState $ \s -> s { Annex.backends = l' }
-		return l'
-	standard = parseBackendList <$> getConfig (annexConfig "backends") ""
-	parseBackendList [] = list
-	parseBackendList s = map lookupBackendName $ words s
+	gen [] = list
+	gen l = map lookupBackendName l
 
 {- Generates a key for a file, trying each backend in turn until one
  - accepts it. -}