Fix cherry picking commits with trailers

When picking commits into a stable release, we now pick commits such that any changelog trailers are retained. This ensures that patch releases still produce changelogs, instead of empty changelogs.

This works as follows:

When a merge request includes no changelog commits, we just cherry pick the merge commit.

When a merge request includes a single changelog commit, we pick that commit directly and retain the Git trailers in the new commit message. This depends on gitlab!62481 (merged) and on gitlab!63228 (merged).

When a merge request includes multiple changelog commits, we cherry pick all merge request commits individually in the correct order.

This fixes https://gitlab.com/gitlab-org/release-tools/-/issues/500.