rebase -i: move rebase--helper modes to rebase--interactive

rebase -i: move rebase--helper modes to rebase--interactive

This moves the rebase--helper modes still used by
git-rebase--preserve-merges.sh (`--shorten-ids`, `--expand-ids`,
`--check-todo-list`, `--rearrange-squash` and `--add-exec-commands`) to
rebase--interactive.c.

git-rebase--preserve-merges.sh is modified accordingly, and
rebase--helper.c is removed as it is useless now.

Signed-off-by: Alban Gruin <alban.gruin@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>