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