sequencer: lib'ify walk_revs_populate_todo()

sequencer: lib'ify walk_revs_populate_todo()

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() is the only caller of
walk_revs_populate_todo(), and it already returns errors
appropriately, so its caller must be already prepared to handle error
returns, and with this step, we make it notice an error return from
this function.

So this is a safe conversion to make walk_revs_populate_todo()
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>