2 changelog entries when merging a PR into another branch

When a maintainer merges a community PR into his branch and creates another PR, changelog puts the maintainer into the top-level sections, while the original PR and author is put to Others. I think we should merge entries (e.g. maintainer should create PR with the same title), and have the second PR mentioned as a fixup (by putting a label on the fixup PR?).

No file or test is named. Start by tracing how changelog entries are generated from merged pull requests, then inspect how PR titles and labels could identify a fixup relationship. Done means a community PR merged into another branch produces one merged changelog entry rather than separate maintainer and original-author entries.

Written by the indexing model from the issue text.