tools/batch-rebase: Fix git cherry-pick --continue error handling

FIX

Distinguish between failures in git cherry-pick --continue:
* If the cherry picking is finished, the topic is rebased and there is
  no issue.
* Otherwise, that indicates further conflicts in the topic that need
  fixing as well.

Before this patch, batch-rebase would just silently drop the commits
after the 2nd conflict in a topic.