Ignore reverted commits when generating changelogs

What does this MR do?

When generating changelogs for a range of commits, any commits both added and reverted in that range are ignored. This works by looking for commits with the pattern "This reverts commit X", then ignoring the commits with the mentioned SHA.

Because commits are retrieved in reverse order (= newest first), and revert commits always come after the commit they revert, we can keep processing commits in batches; instead of having to first load all of them in memory. This means the number of Gitaly calls remains the same.

As part of these changes, CommitsWithTrailerFinder is renamed to ChangelogCommitsFinder, as its purpose is now more specific to the process of generating changelogs; instead of being a generic "give me commits with trailer X" finder.

See gitlab-com/gl-infra/delivery#1583 (closed) for more information.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content