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.

Assistant/Threads/Committer.hs

@@ -32,6 +32,7 @@ import Types.KeySource
 import Config
 import Annex.Exception
 import Annex.Content
+import qualified Annex
 
 import Data.Time.Clock
 import Data.Tuple.Utils
@@ -41,9 +42,9 @@ import Data.Either
 {- This thread makes git commits at appropriate times. -}
 commitThread :: NamedThread
 commitThread = NamedThread "Committer" $ do
-	delayadd <- liftAnnex $ do
-		v <- readish <$> getConfig (annexConfig "delayadd") ""
-		maybe delayaddDefault (return . Just . Seconds) v
+	delayadd <- liftAnnex $
+		maybe delayaddDefault (return . Just . Seconds)
+			=<< annexDelayAdd <$> Annex.getConfig
 	runEvery (Seconds 1) <~> do
 		-- We already waited one second as a simple rate limiter.
 		-- Next, wait until at least one change is available for