Merge branch 'pw/rebase-i-orig-head'

Merge branch 'pw/rebase-i-orig-head'

"git rebase -i" did not store ORIG_HEAD correctly.

* pw/rebase-i-orig-head:
  rebase -i: simplify get_revision_ranges()
  rebase -i: use struct object_id when writing state
  rebase -i: use struct object_id rather than looking up commit
  rebase -i: stop overwriting ORIG_HEAD buffer