Review changes link gets added to review comments that were added after PR merge

[fmease]

Obviously, once a PR is merged nobody can push any more commits to it. Still, triagebot currently adds a Review changes link to comments that were posted after merge (which isn't uncommon if the PR introduced correctness or performance regressions).

Example: rust-lang/rust#152901 (comment).

I expected triagebot to ignore merged PRs when it comes to "review changes".

[Urgau]

That shouldn't be too difficult to avoid.