Merge branch 'kz/merge-fail-early-upon-refresh-failure'

When "git merge" sees that the index cannot be refreshed (e.g. due
to another process doing the same in the background), it died but
after writing MERGE_HEAD etc. files, which was useless for the
purpose to recover from the failure.

* kz/merge-fail-early-upon-refresh-failure:
  merge: avoid write merge state when unable to write index