Reduce overhead of moved files

[danyang685]

I'm using canvas_grab v1.7.7.
It cannot track the moved files for now. They should be moved to new place instead of redownloaded.

Thanks!

[skyzh]

I think this is not easy to implement. It may require hashing of every file. That can be time consuming, and I don't think it's necessary to do this in such small project :(