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.

Annex/Content.hs

@@ -35,7 +35,6 @@ import System.IO.Unsafe (unsafeInterleaveIO)
 import Common.Annex
 import Logs.Location
 import qualified Git
-import qualified Git.Config
 import qualified Annex
 import qualified Annex.Queue
 import qualified Annex.Branch
@@ -188,7 +187,7 @@ withTmp key action = do
  - in a destination (or the annex) printing a warning if not. -}
 checkDiskSpace :: Maybe FilePath -> Key -> Integer -> Annex Bool
 checkDiskSpace destination key alreadythere = do
-	reserve <- getDiskReserve
+	reserve <- annexDiskReserve <$> Annex.getConfig
 	free <- liftIO . getDiskFree =<< dir
 	force <- Annex.getState Annex.force
 	case (free, keySize key) of
@@ -396,11 +395,8 @@ saveState :: Bool -> Annex ()
 saveState nocommit = doSideAction $ do
 	Annex.Queue.flush
 	unless nocommit $
-		whenM alwayscommit $
+		whenM (annexAlwaysCommit <$> Annex.getConfig) $
 			Annex.Branch.commit "update"
-  where
-	alwayscommit = fromMaybe True . Git.Config.isTrue
-		<$> getConfig (annexConfig "alwayscommit") ""
 
 {- Downloads content from any of a list of urls. -}
 downloadUrl :: [Url.URLString] -> FilePath -> Annex Bool