sequencer: lib'ify do_pick_commit()

sequencer: lib'ify do_pick_commit()

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

The only two callers of do_pick_commit(), pick_commits() and
single_pick() already check the return value and pass it on to their
callers, so their callers 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 do_pick_commit() callable from
new callers that want it not to die, without changing the external
behaviour of anything existing.

While at it, remove the superfluous space.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>