Merge branch 'nd/worktree' into maint

* nd/worktree:
  setup_git_directory(): fix move to worktree toplevel directory
  update-index: fix worktree setup
  read-tree: setup worktree if merge is required
  grep: fix worktree setup
  diff*: fix worktree setup

diff --cc builtin-grep.c
Simple merge

diff --cc builtin-update-index.c
Simple merge