fromkey, registerurl: Allow urls to be specified instead of keys, and generate URL keys.

This is especially useful because the caller doesn't need to generate valid
url keys, which involves some escaping of characters, and may involve
taking a md5sum of the url if it's too long.

Backend/URL.hs

@@ -31,8 +31,8 @@ backend = Backend
 	}
 
 {- Every unique url has a corresponding key. -}
-fromUrl :: String -> Maybe Integer -> Annex Key
-fromUrl url size = return $ stubKey
+fromUrl :: String -> Maybe Integer -> Key
+fromUrl url size = stubKey
 	{ keyName = genKeyName url
 	, keyBackendName = "URL"
 	, keySize = size