sequencer: lib'ify checkout_fast_forward()

sequencer: lib'ify checkout_fast_forward()

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

The only callers of checkout_fast_forward(), cmd_merge(),
pull_into_void(), cmd_pull() and sequencer's fast_forward_to(),
already check the return value and handle it appropriately. With this
step, we make it notice an error return from this function.

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