remove Eq instance for InodeCache

There are two types of equality here, and which one is right varies,
so this forces me to consider and choose between them.

Based on this, I learned that the commit in git anex sync was
always doing a strong comparison, even when in a repository where
the inodes had changed. Fixed that.

Command/Add.hs

@@ -109,11 +109,10 @@ ingest (Just source) = do
 	backend <- chooseBackend $ keyFilename source
 	k <- genKey source backend
 	cache <- liftIO $ genInodeCache $ contentLocation source
-	case inodeCache source of
-		Nothing -> go k cache
-		Just c
-			| (Just c == cache) -> go k cache
-			| otherwise -> failure
+	case (cache, inodeCache source) of
+		(_, Nothing) -> go k cache
+		(Just newc, Just c) | compareStrong c newc -> go k cache
+		_ -> failure
   where
 	go k cache = ifM isDirect ( godirect k cache , goindirect k cache )