rework Differences data type

Eliminated complexity and future proofed. The most important change is that
all functions over Difference are now total; any Difference that can be
expressed should be handled. Avoids needs for sanity checking of inputs,
and version skew with the future.

Also, the difference.log now serializes a [Difference], not a Differences.
This saves space and keeps it simpler.

Note that [Difference] might contain conflicting differences (eg,
[Version5, Version6]. In this case, one of them needs to consistently win
over the others, probably based on Ord.

Annex/Difference.hs

@@ -31,9 +31,7 @@ import qualified Data.Map as M
 setDifferences :: Annex ()
 setDifferences = do
 	u <- getUUID
-	otherds <- either error return 
-		=<< sanityCheckDifferences . allDifferences
-			<$> recordedDifferences
+	otherds <- allDifferences <$> recordedDifferences
 	ds <- mappend otherds . annexDifferences <$> Annex.getGitConfig
 	when (ds /= mempty) $ do
 		ds'@(Differences l) <- ifM (isJust <$> getVersion)