sequencer: lib'ify save_head()

sequencer: lib'ify save_head()

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 save_head(), 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 save_head() 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>