Update changelog generation.
Update changelog generation.

* Account for new style merge commit titles
* Fetch the actual PR author (no more issues with sending a branch from
  ember-cli/ember-cli)
* Ignore "dependabot" PRs
* Add support for showing commits that are _not_ included in a PR

HeroicEric/ember-cli,ember-cli/ember-cli,elwayman02/ember-cli,thoov/ember-cli,jrjohnson/ember-cli,HeroicEric/ember-cli,buschtoens/ember-cli,balinterdi/ember-cli,buschtoens/ember-cli,thoov/ember-cli,kategengler/ember-cli,jgwhite/ember-cli,elwayman02/ember-cli,HeroicEric/ember-cli,jrjohnson/ember-cli,jgwhite/ember-cli,asakusuma/ember-cli,HeroicEric/ember-cli,raycohen/ember-cli,jgwhite/ember-cli,thoov/ember-cli,balinterdi/ember-cli,thoov/ember-cli,raycohen/ember-cli,asakusuma/ember-cli,jgwhite/ember-cli,ember-cli/ember-cli,ember-cli/ember-cli,kategengler/ember-cli