Merge branch 'ps/reftable-backend'

Integrate the reftable code into the refs framework as a backend.

* ps/reftable-backend:
  refs/reftable: fix leak when copying reflog fails
  ci: add jobs to test with the reftable backend
  refs: introduce reftable backend

diff --cc .github/workflows/main.yml
Simple merge

diff --cc Makefile
Simple merge

diff --cc repository.h
Simple merge