avoid head

Recent ghc has a deprecation warning on it.

This is not an improvement though. I know these cannot fail, but I can't
prove it to ghc.

Assistant/WebApp/Pairing.hs

@@ -35,8 +35,9 @@ newWormholePairingState = newTVarIO M.empty
 addWormholePairingState :: WormholePairingHandle -> WormholePairingState -> IO WormholePairingId
 addWormholePairingState h tv = atomically $ do
 	m <- readTVar tv
-	-- use of head is safe because allids is infinite
-	let i = Prelude.head $ filter (`notElem` M.keys m) allids
+	-- safe because allids is infinite
+	let i = fromMaybe (error "internal") $ 
+		headMaybe $ filter (`notElem` M.keys m) allids
 	writeTVar tv (M.insert i h m)
 	return i
   where

Remote.hs

@@ -473,7 +473,8 @@ claimingUrl = claimingUrl' (const True)
 claimingUrl' :: (Remote -> Bool) -> URLString -> Annex Remote
 claimingUrl' remotefilter url = do
 	rs <- remoteList
-	let web = Prelude.head $ filter (\r -> uuid r == webUUID) rs
+	let web = fromMaybe (error "internal") $ headMaybe $
+		filter (\r -> uuid r == webUUID) rs
 	fromMaybe web <$> firstM checkclaim (filter remotefilter rs)
   where
 	checkclaim = maybe (pure False) (`id` url) . claimUrl