Merge branch 'jn/v0-with-extensions-fix' into master

In 2.28-rc0, we corrected a bug that some repository extensions are honored by mistake even in a version 0 repositories (these configuration variables in extensions.* namespace were supposed to have special meaning in repositories whose version numbers are 1 or higher), but this was a bit too big a change. * jn/v0-with-extensions-fix: repository: allow repository format upgrade with extensions Revert "check_repository_format_gently(): refuse extensions for old repositories"