Do not notify if only rustbot is pinged in a commit message

[blyxyas]

Previously, users were getting warnings for including merge commits to PRs that mentioned only @rustbot (Example). Now it has to have at least one human mentioned in order to trigger a comment.

[Urgau]

A more principal fix would be to ignore subtree syncs, probably by adding a config on [no-mentions] (à la exclude-titles = ["subtree update"]).

Would you be willing to implement this instead? I can provide guidance. If not, that's okay, we can merge this PR in the meantime.

[blyxyas]

Yes I'd be willing to implement such a change, triagebot is a project I like working on.

Although I'm not sure how many titles that aren't subtree syncs (and thus, contain merge commits mentioning rustbot) would benefit from disabling the no-mentions check.

[Urgau]

I proposed title based exclusion because that's what we currently do for the [no-merges] handler (example in r-l/r) and I think we are kind-of forced to do it for subtrees.

I don't think only excluding rustbot would work for subtrees, as merge commits often contain the complete description which can contain arbitrary mentions, which I don't think you would want them to be warn either.

As for other PRs that aren't subtree sync, we may still want to exclude rustbot (and maybe bors as well), although the usefulness of not linting for those PRs is less clear to me.