Warn on new or deleted files

[asmeurer]

If a commit introduces a new file or deletes a file, we could have the bot warn about it. That way people will notice issues like sympy/sympy#18488. We could also make it fail in such cases, unless someone with push access whitelists the files somehow, e.g., by mentioning the bot and listing the files. CC @oscarbenjamin

[asmeurer]

Here is a proof-of-concept. #77

For now it just posts a comment, like asmeurer/GitHub-Issues-Test#44 (comment), but we could easily make it post a failing status and require someone with push access to @mention the bot to whitelist the files.

[oscarbenjamin]

I think this is definitely a good idea. It would be better to try this out for a while before having it fail the PR.

[oscarbenjamin]

I've merged the PR. I guess we can leave this open for now until we're sure we're happy with it. Looks like this issue will get notified every time the bot posts though...

[asmeurer]

Yeah, I noticed that. But I think it's useful because it can make it easy to find all the instances of this. It looks like there are already a few so far, but all are intended file additions.

[asmeurer]

This needs to be fixed to skip merge commits. See sympy/sympy#18681

[asmeurer]

#81