rebase -i: implement the logic to initialize $revisions in C

rebase -i: implement the logic to initialize $revisions in C

This rewrites the part of init_revisions_and_shortrevisions() needed by
`--make-script` from shell to C.  The new version is called
get_revision_ranges(), and is a static function inside of
rebase--helper.c.  As this does not initialize $shortrevision, the
original shell version is not yet stripped.

Unlike init_revisions_and_shortrevisions(), get_revision_ranges()
doesn’t write $squash_onto to the state directory, it’s done by the
handler of `--make-script` instead.

Finally, this drops the $revision argument passed to `--make-script` in
git-rebase--interactive.sh, and rebase--helper is changed accordingly.

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