sequencer (rebase -i): allow rescheduling commands

sequencer (rebase -i): allow rescheduling commands

The interactive rebase has the very special magic that a cherry-pick
that exits with a status different from 0 and 1 signifies a failure to
even record that a cherry-pick was started.

This can happen e.g. when a fast-forward fails because it would
overwrite untracked files.

In that case, we must reschedule the command that we thought we already
had at least started successfully.

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