Merge branch 'ps/fsync-refs'

Updates to refs traditionally weren't fsync'ed, but we can
configure using core.fsync variable to do so.

* ps/fsync-refs:
  core.fsync: new option to harden references

diff --cc cache.h
Simple merge

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

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