Add support to changelog-draft for non-squash merge strategies

We're currently looking up a commit's associated PR by searching for a PR with a matching `merge_commit_sha` field. This works well for squash-merged PRs, but in other cases, it leaves us with a bunch of unassociated commits (i.e. the ones that aren't the merge commit or the head of the rebased-and-ff-merged branch).

Start by locating the changelog-draft code that searches for pull requests using merge_commit_sha, then trace how associated commits are collected. Review the GitHub pull request data and merge strategies involved. Done means non-squash merges no longer leave applicable commits unassociated; add or update coverage if the project has tests for this behavior.

Written by the indexing model from the issue text.