Merge branch 'js/rebase-autosquash-double-fixup-fix'

"rebase -i" segfaulted when rearranging a sequence that has a
fix-up that applies another fix-up (which may or may not be a
fix-up of yet another step).

* js/rebase-autosquash-double-fixup-fix:
  rebase --autosquash: fix a potential segfault

diff --cc sequencer.c
Simple merge

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