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.

1 files changed, 2 insertions, 2 deletions

diff --git a/Backend.hs b/Backend.hs
index a09fc0e99..9a40e5459 100644
--- a/Backend.hs
+++ b/Backend.hs
@@ -26,12 +26,12 @@ import Types.Key
 import qualified Types.Backend as B
 
 -- When adding a new backend, import it here and add it to the list.
-import qualified Backend.WORM
 import qualified Backend.SHA
+import qualified Backend.WORM
 import qualified Backend.URL
 
 list :: [Backend Annex]
-list = Backend.WORM.backends ++ Backend.SHA.backends ++ Backend.URL.backends
+list = Backend.SHA.backends ++ Backend.WORM.backends ++ Backend.URL.backends
 
 {- List of backends in the order to try them when storing a new key. -}
 orderedList :: Annex [Backend Annex]