#9002 List the pull request's changelog entries atop the semantic diff comment

The heading repeated the last commit's sha and a time. The comment now opens with a Changelog section listing the entries the pull request adds, each led by its category's emoji as in the changelog, and noting breaking commits without a breaking entry or the other way round; the Semantic Diff heading follows.

The budget shortened only commits, so fragments of more than the budget would have pushed the comment past GitHub's limit (Codex). The changelog takes at most a quarter of the budget; entries past it are counted.