handlers: add no merge policy notifications

[davidtwco]

Inspired by discussion on Zulip.

Add a handler for issue events that checks whether a merge commit has been added to the pull request and informs the user of the project's no merge policy.

cc @Mark-Simulacrum

[davidtwco]

This passes a cargo check but I haven't yet had the time to set up a proper environment to check it actually works, so opening as a draft.

[Mark-Simulacrum]

I wonder if we'd get a Github-native warning by adding "require linear history" to the branch protections on rust-lang/rust or whatever other repositories. Maybe worth experimenting with? We obviously don't actually have a linear history, so we'd need to bypass that in bors, but maybe then PRs would get warnings of some kind?

(I think this is probably still worth doing, and we might want a status check of some kind that prevents bors merges too...)

[davidtwco]

I wonder if we'd get a Github-native warning by adding "require linear history" to the branch protections on rust-lang/rust or whatever other repositories. Maybe worth experimenting with?

I've received feedback that GitHub will offer a "fix this conflict" button of some sort that adds a merge commit, which is immediately incorrect, so we could reduce the incidences of this happening by preventing GitHub from offering that option.

[Mark-Simulacrum]

@davidtwco this is still marked as a draft, is that state accurate? Or are we good to go?

[davidtwco]

@davidtwco this is still marked as a draft, is that state accurate? Or are we good to go?

I haven't taken the time to set up a development environment for this locally and actually check it does what I expect it to (just because that seems like something that I'll need to block out some time to do).

[Mark-Simulacrum]

Oh. Well, feel free to if you want, but we can also merge this and then enable it (perhaps not on rust-lang/rust) and test it that way. Just let me know if you'd prefer that; it's pretty typical of triagebot PRs.

[davidtwco]

Oh. Well, feel free to if you want, but we can also merge this and then enable it (perhaps not on rust-lang/rust) and test it that way. Just let me know if you'd prefer that; it's pretty typical of triagebot PRs.

Sorry for the delay here, we can test it after merging I think.