sequencer: lib'ify sequencer_pick_revisions()

sequencer: lib'ify sequencer_pick_revisions()

Instead of dying there, let the caller high up in the callchain notice
the error and handle it (by dying, still).

The function sequencer_pick_revisions() has only two callers,
cmd_revert() and cmd_cherry_pick(), both of which check the return
value and react appropriately upon errors.

So this is a safe conversion to make sequencer_pick_revisions()
callable from new callers that want it not to die, without changing
the external behaviour of anything existing.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>