This is heavily based on Stephan Beyer's git sequencer rewrite of rebase-i-p.
Each commit is still found by rev-list UPSTREAM..HEAD, but a commit is only
included in todo if at least one its parents has been marked for rewriting.
Signed-off-by: Stephen Haberman <stephen@exigencecorp.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>