This was failing for me locally so I've made some fixes:
- remove the `markdown` flag (as it's the only path now)
- refactor the generation to not use intermediate variables
- discard more weird cases rather than erroring
- exclude @Homebrew changes (e.g. from bots) because we don't care about
these in the release notes
I forgot until I started to work on the 3.0.0 release that there was an
added step I would do for a major/minor release:
```
brew release-notes --markdown 2.7.0 | pbcopy
```
This would generate for 3.0.0/2.8.0 the release notes since 2.7.0 for
editing down into a blog post.
Add this behaviour to `brew release` and, because we're doing it
programmatically anyway, take this chance to also reject Dependabot
lines and sort them for easier grouping by pull request subject.
