[util] bitstream_bisect tool should warn when venturing past a "barrier" commit

> In retrospect my bisection is affected by stale bitstreams being cached because of this issue: #16925
>
> So it may be ROM changes in commits preceding this one that are then provided in the cache when this one ran through CI.

Thanks for pointing that out, @drewmacrae. I feel like it might make sense to hardcode a "barrier" commit in the bitstream bisect tool. If bisecting would venture past that barrier, the script should print a warning. Every time we discover a bug like #16925 that invalidates prior cached bitstreams like this, we should bump the barrier commit.

_Originally posted by @dmcardle in https://github.com/lowRISC/opentitan/issues/16576#issuecomment-1363184871_