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Doesn't `pulumi preview --diff` work for this case?

I understand I'm not answering your precise question, but I thought that maybe you're looking for something simpler.

`pulumi preview --diff` compares current pulumi program against the current state of the stack. This is useful and necessary, but I additionally want to have something else:
```git checkput sunnyday
pulumi dumptostack -o /tmp/sunnyday.stack
git checkput rainyday
pulumi dumptostack -o /tmp/rainyday.stack
pulumi diffstacks /tmp/sunnyday.stack /tmp/rainyday.stack```
or (which would probably fit into pulumi model more realistically):
```pulumi stack init fake-rainyday
git checkput rainyday
pulumi fake-up --yes
git checkput sunnyday
pulumi preview -s fake-rainyday --diff```

Rationale for this feature is to introduce policies / automatic checks on changes made to the pulumi programs, and have them validated enforced in pull requests which modify pulumi programs.

I view policy decision making as a function of a change in the goal state not a function of a difference between goal state and a current state of the system.

here's my hacky attempt to do it, but I'd like  something cleaner and prettier in terms of output:
```pulumi stack init empty-stack
git checkout rainyday
pulimi preview --diff -s empty-stack &gt; /tmp/rainy.txt
git checkout sunnyday
pulimi preview --diff -s empty-stack &gt; /tmp/sunny.txt
diff -p /tmp/{rainy,sunny}.txt```

But are you storing your stacks inside git, too?

Actually, nevermind. I've just noticed you're creating an empty stack just for diffing.