triagebot: Amend a review to include a link to what was changed since (#15580)

[This
feature](https://forge.rust-lang.org/triagebot/review-changes-since.html)
adds a handy link to each review message (it doesn't create a new
comment) that allows the reviewer or other people to check what has been
changed in the PR since the review took place.

changelog: none

r? flip1995

Author: flip1995

triagebot.toml

@@ -52,6 +52,9 @@ labels = ["S-waiting-on-concerns"]
 # Show differences when a PR is rebased
 [range-diff]
 
+# Amend a review to include a link to what was changed since the review
+[review-changes-since]
+
 [assign]
 contributing_url = "https://github.com/rust-lang/rust-clippy/blob/master/CONTRIBUTING.md"
 users_on_vacation = [