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

[anandthakker]

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).

[1ec5]

rebased-and-ff-merged branch

👍 This is a somewhat common scenario in the mapbox-gl-native repository.