sequencer: lib'ify prepare_revs()

sequencer: lib'ify prepare_revs()

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

The only caller of prepare_revs(), walk_revs_populate_todo() was just
taught to return errors, after verifying that its callers are 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 prepare_revs() 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>