use SHA256 by default

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

I feel that SHA256 is a better default for most people, as long as their
systems are fast enough that checksumming their files isn't a problem.
git-annex should default to preserving the integrity of data as well as git
does. Checksum backends also work better with editing files via
unlock/lock.

I considered just using SHA1, but since that hash is believed to be somewhat
near to being broken, and git-annex deals with large files which would be a
perfect exploit medium, I decided to go to a SHA-2 hash.

SHA512 is annoyingly long when displayed, and git-annex displays it in a
few places (and notably it is shown in ls -l), so I picked the shorter
hash. Considered SHA224 as it's even shorter, but feel it's a bit weird.

I expect git-annex will use SHA-3 at some point in the future, but
probably not soon!

Note that systems without a sha256sum (or sha256) program will fall back to
defaulting to SHA1.

Backend/SHA.hs

@@ -16,12 +16,12 @@ import qualified Build.SysConfig as SysConfig
 
 type SHASize = Int
 
+-- order is slightly significant; want SHA256 first, and more general
+-- sizes earlier
 sizes :: [Int]
-sizes = [1, 256, 512, 224, 384]
+sizes = [256, 1, 512, 224, 384]
 
 backends :: [Backend Annex]
--- order is slightly significant; want sha1 first, and more general
--- sizes earlier
 backends = catMaybes $ map genBackend sizes ++ map genBackendE sizes
 
 genBackend :: SHASize -> Maybe (Backend Annex)