Missing/wrong pull requests for commits

Describe the issue

The current approach to associate commits with the pull requests is the following:

1. Fetch the commits.
2. Fetch the pull requests.
3. Check if the merge_commit_sha of the pull request matches the SHA of the commit.

github_commits.for_each(|commit| {
		let pull_request = github_pull_requests
			.iter()
			.find(|pr| pr.merge_commit_sha == Some(commit.sha.clone()));
    });

This poses a couple of problems when the pull request is not squashed but rebased.

Take #360 for example (it is rebased). git cliff -c github outputs:

* fix: allow version bump with a single previous release by @orhun in #360
* fix: allow version bump with a single previous release by @nappa85

However, it should be:

* fix: allow version bump with a single previous release by @nappa85 in #360

Additional context

I don't know how to fix this yet since it is the limitation of the data we get from the GitHub API. In the perfect world, /commits endpoint would also return the associated pull request number so that we don't need to fetch the pull requests and do some hacky comparison.

Alternatively, we could use <commit>/pull API but that just means sending a request for each commit which adds up to a lot.