rebase: fill `squash_onto' in get_replay_opts()

rebase: fill `squash_onto' in get_replay_opts()

When sequencer_continue() is called by complete_action(), `opts' has
been filled by get_replay_opts().  Currently, it does not initialise the
`squash_onto' field (used by the `--root' mode), only
read_populate_opts() does.  It’s not a problem yet since
sequencer_continue() calls it before pick_commits(), but it would lead
to incorrect results once complete_action() is modified to call
pick_commits() directly.

Let’s change that.

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