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

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"