Don't create empty commits in the instruction ref

If the user asks to edit the instruction sheet but does not actually
change anything, then git-integration currently creates an empty commit
on the instruction sheet branch.

This is unnecessary and makes it harder to find genuine changes to the
instruction sheet, so detect this situation and do not create a new
commit if it would be empty.

Signed-off-by: John Keeping <john@keeping.me.uk>
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.