forbid upgrading an external WC to mismatch its parent WC format

Daniel Shahaf wrote, in dev@ email "multi-wc-format: upgrading externals" (https://mail-archives.apache.org/mod_mbox/subversion-dev/202202.mbox/%3C20220219144836.GA2817%40tarpaulin.shahaf.local2%3E):

multi-wc-format/BRANCH-README mentioned this:

> [*] New externals working copies must inherit the format from their
> parent working copy, because mixed-format working copies are a) a
> Bad Thing, and b) defeat the purpose of this feature, which is
> support for multiple versions of the client in the same working
> copy.

Upgrading a parent working copy upgrades external wc's too. However,
upgrading an external succeeds. Judging by the quoted remark, should
«svn upgrade --compatible-version=$N /path/to/external» error out unless
the external's parent working copy is already at version $V?