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

diff --cc cache.h
Simple merge

diff --cc setup.c
Simple merge

diff --cc t/t1302-repo-version.sh
Simple merge