When checking that an url has a key, verify that the Content-Length, if available, matches the size of the key.

If there's no Content-Length, or the key has no size, this check is not
done, but it should happen most of the time, and protect against web
content that has changed.

Remote/Git.hs

@@ -28,6 +28,7 @@ import qualified Utility.Url as Url
 import Utility.TempFile
 import Config
 import Init
+import Types.Key
 
 remote :: RemoteType
 remote = RemoteType {
@@ -143,7 +144,8 @@ inAnnex r key
 			where
 				go e [] = return $ Left e
 				go _ (u:us) = do
-					res <- catchMsgIO $ Url.exists u
+					res <- catchMsgIO $
+						Url.check u (keySize key)
 					case res of
 						Left e -> go e us
 						v -> return v