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>