hoist next_save to all saves

Now we can rely on its state even when we don't rely on autosave.

edit.lua

@@ -140,7 +140,6 @@ end
 function edit.update(State, dt)
   if State.next_save and State.next_save < Current_time then
     save_to_disk(State)
-    State.next_save = nil
   end
 end
 

file.lua

@@ -41,6 +41,7 @@ function save_to_disk(State)
     outfile:write('\n')
   end
   outfile:close()
+  State.next_save = nil
 end
 
 -- for tests