Commits that revert other commits don't get copied into the Unreleased section

If a changelog entry is added to the changelog in a previous release with an attached PR and then another commit is created which reverts that commit, it will not be copied to the Unreleased section. This happens because `auto-changelog` thinks that it's already been added (because the title of the revert commit includes the original PR number). (See [example revert commit](https://github.com/MetaMask/core/commit/3f15e1ed1f26ced9f021b3c08a3f26feec7d6c18).)

Parsing the PR number out of the changelog entry would fix this. This may be related to https://github.com/MetaMask/auto-changelog/issues/150 in that sense.

Start by tracing the auto-changelog logic that decides whether a changelog entry is already present, focusing on how it parses PR numbers from entries and revert commit titles. Reproduce the behavior using the linked example revert commit and verify that a reverted entry is copied into the Unreleased section without being treated as already added.

Written by the indexing model from the issue text.