"git rev-list --bisect --first-parent" does not work (yet) and can
even cause SEGV; forbid it.  "git log --bisect --first-parent"
would not be useful until "git bisect --first-parent" materializes,
so it is also forbidden for now.

* kd/rev-list-bisect-first-parent:
  rev-list: refuse --first-parent combined with --bisect