sequencer: do not die() in do_pick_commit()

sequencer: do not die() in do_pick_commit()

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

The eventual caller of do_pick_commit() is sequencer_pick_revisions(),
which already relays a reported error from its helper functions
(including this one), and both of its two callers know how to react to
a negative return correctly.

So this makes do_pick_commit() 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>