sequencer: lib'ify fast_forward_to()

sequencer: lib'ify fast_forward_to()

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 fast_forward_to(), do_pick_commit() already checks
the return value and passes it on to its callers, 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 fast_forward_to() 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>