Merge branch 'jk/reject-newer-extensions-in-v0' into master

With the base fix to 2.27 regresion, any new extensions in a v0
repository would still be silently honored, which is not quite
right.  Instead, complain and die loudly.

* jk/reject-newer-extensions-in-v0:
  verify_repository_format(): complain about new extensions in v0 repo

t/t1302-repo-version.sh

@@ -87,6 +87,9 @@ allow 1
 allow 1 noop
 abort 1 no-such-extension
 allow 0 no-such-extension
+allow 0 noop
+abort 0 noop-v1
+allow 1 noop-v1
 EOF
 
 test_expect_success 'precious-objects allowed' '