Merge branch 'kn/reflog-migration-fix-fix'

Fix bugs in an earlier attempt to fix "git refs migration".

* kn/reflog-migration-fix-fix:
  refs/reftable: fix uninitialized memory access of `max_index`
  reftable: write correct max_update_index to header

diff --cc refs.c
Simple merge

diff --cc refs/refs-internal.h
Simple merge

diff --cc refs/reftable-backend.c
Simple merge

diff --cc t/t1460-refs-migrate.sh
Simple merge