coqbot should be able to do traditional delta debugging on a PR

[JasonGross]

I was reading up on delta debugging and found that one use case is to find the minimal set of changes in a diff that can be removed to restore working behavior. When we get a fast enough file (such as https://github.com/coq/coq/pull/14727#issuecomment-957082119 , perhaps in conjunction with #177), it would be cool to have a mode that first bisects the change over the commits in the PR, and then applies delta debugging to find a minimal diff that's important to the bug