Phillip Wood [off-list ref] writes:

quoted

The relevance is this: _iff_ we want to prevent `git bisect` from
running
in a subdirectory under the premise that that subdirectory might need to
be removed, then why don't we prevent `git rebase` from running in a
subdirectory when that command is equally likely to remove that
subdirectory?

That's a good point, I'd completely missed the analogy with `rebase --exec`

I concur.  Perhaps we should make rebase a bit more careful.