We try to stick to a spack release with selected cherry-picks. Occasionally, spack has a treewide commit that breaks this because all subsequent cherry-picks need the treewide commit to apply successfully, and applying the treewide commit as a cherry-pick would require essentially every commit since the release. This PR adds the ability to apply partial cherry-picks, restricted to just the specified files. This allows is to apply the treewide commit on only the files that need it.