verify_repository_format(): complain about new extensions in v0 repo

verify_repository_format(): complain about new extensions in v0 repo

We made the mistake in the past of respecting extensions.* even when the
repository format version was set to 0. This is bad because forgetting
to bump the repository version means that older versions of Git (which
do not know about our extensions) won't complain. I.e., it's not a
problem in itself, but it means your repository is in a state which does
not give you the protection you think you're getting from older
versions.

For compatibility reasons, we are stuck with that decision for existing
extensions. However, we'd prefer not to extend the damage further. We
can do that by catching any newly-added extensions and complaining about
the repository format.

Note that this is a pretty heavy hammer: we'll refuse to work with the
repository at all. A lesser option would be to ignore (possibly with a
warning) any new extensions. But because of the way the extensions are
handled, that puts the burden on each new extension that is added to
remember to "undo" itself (because they are handled before we know
for sure whether we are in a v1 repo or not, since we don't insist on a
particular ordering of config entries).

So one option would be to rewrite that handling to record any new
extensions (and their values) during the config parse, and then only
after proceed to handle new ones only if we're in a v1 repository. But
I'm not sure if it's worth the trouble:

  - ignoring extensions is likely to end up with broken results anyway
    (e.g., ignoring a proposed objectformat extension means parsing any
    object data is likely to encounter errors)

  - this is a sign that whatever tool wrote the extension field is
    broken. We may be better off notifying immediately and forcefully so
    that such tools don't even appear to work accidentally.

The only downside is that fixing the situation is a little tricky,
because programs like "git config" won't want to work with the
repository. But:

  git config --file=.git/config core.repositoryformatversion 1

should still suffice.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>