sequencer: lib'ify create_seq_dir()

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 create_seq_dir(), sequencer_pick_revisions() can already return errors, 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 create_seq_dir() 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>