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