Merge branch 'ag/rebase-p'

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