addurl: --fast can be used to avoid immediately downloading the url.

The tricky part about this is that to generate a key, the file must be
present already. Worked around by adding (back) an URL key type, which
is used for addurl --fast.

Backend/URL.hs

@@ -0,0 +1,28 @@
+{- git-annex "URL" backend -- keys whose content is available from urls.
+ -
+ - Copyright 2011 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Backend.URL (
+	backends,
+	fromUrl
+) where
+
+import Types.Backend
+import Types.Key
+import Types
+
+backends :: [Backend Annex]
+backends = [backend]
+
+backend :: Backend Annex
+backend = Types.Backend.Backend {
+	name = "URL",
+	getKey = const (return Nothing),
+	fsckKey = const (return True)
+}
+
+fromUrl :: String -> Key
+fromUrl url = stubKey { keyName = url, keyBackendName = "URL" }