Oh I see. That's the problem. I tried repositor in the diffusion, it works well.
My fault, thank you guys. This task could be close.

I did't add my git repositor into Diffusion. Does that matter?

And here is the --trace information

I create a new empty project on my gitlab and clone it to local. And add .arcconfig file then commit it. After that I ran the arc diff command and it create a new revision on phabricator.
When I want to update the existing revision, I modify a README.md file and ran arc diff --update <Revision Number>. It's create a new revision every single time.
I'm using Mac, and it's the same on my mate's Windows computer. I don't know why.
Maybe I didn't configure currectly. So I want to know where was wrong. Here is my phabricator information.