Merge branch 'no-empty-commits' into pu

* branch no-empty-commits:
  Don't create empty commits in the instruction ref
  Add a "fixup" instruction
  status: include SHA-1 of merge to integration branch
  t3002: add a test with multiple up-to-date branches
  status: Extract status_base as a separate step
  WIP
  Add --skip option

Don't insert empty commits into the instruction sheet branch when the
user closes their editor without changing the instruction sheet.