Merge branch 'jc/rebase-detach-fix'

"git rebase $base $non_branch_commit", when $base is an ancestor or
the $non_branch_commit, modified the current branch, which has been
corrected.

* jc/rebase-detach-fix:
  rebase: set REF_HEAD_DETACH in checkout_up_to_date()
  rebase: use test_commit helper in setup