remove checksum from WORM

with size and mtime in the key, it's redundant

Backend/WORM.hs

@@ -6,7 +6,6 @@ module Backend.WORM (backend) where
 import Control.Monad.State
 import System.FilePath
 import System.Posix.Files
-import Data.Digest.Pure.SHA -- slow, but we only checksum filenames
 import qualified Data.ByteString.Lazy.Char8 as B
 
 import qualified Backend.File
@@ -18,25 +17,18 @@ backend = Backend.File.backend {
 	getKey = keyValue
 }
 
--- A SHA1 of the basename of the filename, plus the file size and
--- modification time, is used as the unique part of the key. That
--- allows multiple files with the same names to have different keys,
+-- The key is formed from the file size, modification time, and the
+-- basename of the filename.
+--
+-- That allows multiple files with the same names to have different keys,
 -- while also allowing a file to be moved around while retaining the
 -- same key.
--- 
--- The file size and modification time are also included in the key,
--- unhashed. This could be used as a sanity check.
--- 
--- The basename of the filename is also included in the key, so it's clear
--- what the original filename was when a user sees the value.
 keyValue :: FilePath -> Annex (Maybe Key)
 keyValue file = do
 	stat <- liftIO $ getFileStatus file
 	return $ Just $ Key ((name backend), key stat)
 	where
- 		key stat = (checksum $ uniqueid stat) ++ sep ++
-				uniqueid stat ++ sep ++ base
-		checksum s = show $ sha1 $ B.pack s
+ 		key stat = uniqueid stat ++ sep ++ base
 		uniqueid stat = (show $ modificationTime stat) ++ sep ++
 			(show $ fileSize stat)
 		base = takeFileName file