Many projects don’t really think of Pull Requests
as queues of perfect patches
that should apply cleanly in order,
as most mailing list-based projects
think of patch series contributions.
Most GitHub projects think about Pull Request branches
as iterative conversations around a proposed change,
culminating in a unified diff that is applied by merging.
Pushing the “Merge” button on the site purposefully creates a merge commit that references the Pull Request so that it’s easy to go back and research the original conversation if necessary.