Also show warnings when PRs are updated

[Urgau]

This PR tweaks the assign handler to also show warnings when PRs are updated, instead of just doing it when a PR is opened.

Fixes #1831
r? @ehuss

[ehuss]

This looks like it will warn on every push, is that right? If possible I'd like to avoid that (maybe with IssueData?).

[Urgau]

That's a good observation, it would indeed post unnecessary comments.

As suggested I have used IssueData with some state to keep track of the last warnings to avoid posting unnecessary comments. I haven't tested it, but it's very similar to the other avoid comment logic.