rebase -i: implement the main part of interactive rebase as a builtin

rebase -i: implement the main part of interactive rebase as a builtin

This rewrites the part of interactive rebase which initializes the
basic state, make the script and complete the action, as a buitin, named
git-rebase--interactive2 for now.  Others modes (`--continue`,
`--edit-todo`, etc.) will be rewritten in the next commit.

git-rebase--interactive.sh is modified to call git-rebase--interactive2
instead of git-rebase--helper.

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