Merge branch 'ow/rebase-verify-insn-fmt-before-initializing-state'

"git rebase -i" with bogus rebase.instructionFormat configuration
failed to produce the todo file after recording the state files,
leading to confused "git status"; this has been corrected.

* ow/rebase-verify-insn-fmt-before-initializing-state:
  rebase: write script before initializing state

diff --cc builtin/rebase.c
Simple merge

diff --cc t/t3415-rebase-autosquash.sh
Simple merge