Merge branch 'ph/rebase-preserve-all-merges'

An earlier change to add --keep-empty option broke "git rebase
--preserve-merges" and lost merge commits that end up being the
same as its parent.

* ph/rebase-preserve-all-merges:
rebase --preserve-merges: keep all merge commits including empty ones

diff --cc git-rebase--interactive.sh
Simple merge