sequencer: change the way skip_unnecessary_picks() returns its result

sequencer: change the way skip_unnecessary_picks() returns its result

Instead of skip_unnecessary_picks() printing its result to stdout, it
returns it into a struct object_id, as the rewrite of complete_action()
(to come in the next commit) will need it.

rebase--helper then is modified to fit this change.

Signed-off-by: Alban Gruin <redacted>
Signed-off-by: Junio C Hamano <redacted>