rebase -i: rewrite the rest of init_revisions_and_shortrevisions() in C

rebase -i: rewrite the rest of init_revisions_and_shortrevisions() in C

This rewrites the part of init_revisions_and_shortrevisions() needed by
`--complete-action` (which initialize $shortrevisions) from shell to C.

When `upstream` is empty, it means that the user launched a `rebase
--root`, and `onto` contains the ID of an empty commit.  As a range
between an empty commit and `head` is not really meaningful, `onto` is
not used to initialize `shortrevisions` in this case.

The corresponding arguments passed to `--complete-action` are then
dropped, and init_revisions_and_shortrevisions() is stripped from
git-rebase--interactive.sh

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