Merge branch 'jk/fsck-indices-in-worktrees'

"git fsck" learned to check the index files in other worktrees, just like "git gc" honors them as anchoring points. * jk/fsck-indices-in-worktrees: fsck: check even zero-entry index files fsck: mention file path for index errors fsck: check index files in all worktrees fsck: factor out index fsck