GitHub action to show generated diff preview (#778)

* Add `mk-generated.sh`

`tools/mk-generated.sh` will run code generation and create an orphan branch containing the codegen results.

* Add GitHub action to push the generated code to a branch

* Add cleanup job

* backout accidental changes

* Apply suggestions from code review

Co-authored-by: Zelda Hessler <zelda.hessler@pm.me>

Co-authored-by: Zelda Hessler <zelda.hessler@pm.me>