SHA256E is new default backend

The default backend used when adding files to the annex is changed from
SHA256 to SHA256E, to simplify interoperability with OSX, media players,
and various programs that needlessly look at symlink targets.

To get old behavior, add a .gitattributes containing: * annex.backend=SHA256

Backend/SHA.hs

@@ -1,4 +1,4 @@
-{- git-annex SHA backend
+{- git-annex SHA backends
  -
  - Copyright 2011,2012 Joey Hess <joey@kitenet.net>
  -
@@ -19,13 +19,14 @@ import qualified Data.ByteString.Lazy as L
 
 type SHASize = Int
 
--- order is slightly significant; want SHA256 first, and more general
--- sizes earlier
+{- Order is slightly significant; want SHA256 first, and more general
+ - sizes earlier. -}
 sizes :: [Int]
 sizes = [256, 1, 512, 224, 384]
 
+{- The SHA256E backend is the default. -}
 backends :: [Backend]
-backends = catMaybes $ map genBackend sizes ++ map genBackendE sizes
+backends = catMaybes $ map genBackendE sizes ++ map genBackend sizes
 
 genBackend :: SHASize -> Maybe Backend
 genBackend size = Just $ Backend