handle really long urls

Using the whole url as a key can make the filename too long. Truncate
and use a md5sum for uniqueness if necessary.

Backend/URL.hs

@@ -10,6 +10,8 @@ module Backend.URL (
 	fromUrl
 ) where
 
+import Data.Hash.MD5
+
 import Common.Annex
 import Types.Backend
 import Types.Key
@@ -26,7 +28,14 @@ backend = Backend {
 
 fromUrl :: String -> Maybe Integer -> Key
 fromUrl url size = stubKey
-	{ keyName = url
+	{ keyName = key
 	, keyBackendName = "URL"
 	, keySize = size
 	}
+	where
+		-- when it's not too long, use the url as the key name
+		-- 256 is the absolute filename max, but use a shorter
+		-- length because this is not the entire key filename.
+		key
+			| length url < 128 = url
+			| otherwise = take 128 url ++ "-" ++ md5s (Str url)