separate source of content from the filename associated with the key when generating a key

This already made migrate's code a lot simpler.

Types/Backend.hs

@@ -2,7 +2,7 @@
  -
  - Most things should not need this, using Types instead
  -
- - Copyright 2010 Joey Hess <joey@kitenet.net>
+ - Copyright 2010,2012 Joey Hess <joey@kitenet.net>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
@@ -11,11 +11,18 @@ module Types.Backend where
 
 import Types.Key
 
+{- The source used to generate a key. The location of the content
+ - may be different from the filename associated with the key. -}
+data KeySource = KeySource
+	{ keyFilename :: FilePath
+	, contentLocation :: FilePath
+	}
+
 data BackendA a = Backend {
 	-- name of this backend
 	name :: String,
-	-- converts a filename to a key
-	getKey :: FilePath -> a (Maybe Key),
+	-- gets the key to use for a given content
+	getKey :: KeySource -> a (Maybe Key),
 	-- called during fsck to check a key, if the backend has its own checks
 	fsckKey :: Maybe (Key -> FilePath -> a Bool)
 }