Separate "rebase -p" codepath out of "rebase -i" implementation to
slim down the latter and make it easier to manage.

* ag/rebase-p:
  rebase: remove -p code from git-rebase--interactive.sh
  rebase: use the new git-rebase--preserve-merges.sh
  rebase: strip unused code in git-rebase--preserve-merges.sh
  rebase: introduce a dedicated backend for --preserve-merges